[decimal] Implement exp() for BigDecimal (#74)

Author: forfudan

README.md

@@ -1,9 +1,19 @@
-# DeciMojo
+# DeciMojo <!-- omit in toc -->
 
 A comprehensive decimal and integer mathematics library for [Mojo](https://www.modular.com/mojo).
 
 **[中文·漢字»](https://zhuyuhao.com/decimojo/docs/readme_zht.html)**　|　**[Repository on GitHub»](https://github.com/forfudan/decimojo)**  | **[Changelog](https://zhuyuhao.com/decimojo/docs/changelog.html)**
 
+- [Overview](#overview)
+- [Installation](#installation)
+- [Quick start](#quick-start)
+- [Objective](#objective)
+- [Nomenclature](#nomenclature)
+- [Status](#status)
+- [Tests and benches](#tests-and-benches)
+- [Citation](#citation)
+- [License](#license)
+
 ## Overview
 
 DeciMojo provides a comprehensive decimal and integer mathematics library for Mojo, delivering exact calculations for financial modeling, scientific computing, and applications where floating-point approximation errors are unacceptable. Beyond basic arithmetic, the library includes advanced mathematical functions with guaranteed precision.
@@ -168,15 +178,15 @@ The name ultimately emphasizes our mission: bringing precise, reliable decimal c
 
 Rome wasn't built in a day. DeciMojo is currently under active development. For the 128-bit `Decimal` type, it has successfully progressed through the **"make it work"** phase and is now well into the **"make it right"** phase with many optimizations already in place. Bug reports and feature requests are welcome! If you encounter issues, please [file them here](https://github.com/forfudan/decimojo/issues).
 
-### Make it Work ✅ (COMPLETED)
+### Make it Work ✅ (COMPLETED)  <!-- omit in toc -->
 
 - Core decimal implementation with a robust 128-bit representation (96-bit coefficient + 32-bit flags)
 - Comprehensive arithmetic operations (+, -, *, /, %, **) with proper overflow handling
 - Type conversions to/from various formats (String, Int, Float64, etc.)
 - Proper representation of special values (NaN, Infinity)
 - Full suite of comparison operators with correct decimal semantics
 
-### Make it Right 🔄 (MOSTLY COMPLETED)
+### Make it Right 🔄 (MOSTLY COMPLETED) <!-- omit in toc -->
 
 - Reorganized codebase with modular structure (decimal, arithmetics, comparison, exponential).
 - Edge case handling for all operations (division by zero, zero to negative power).
@@ -186,7 +196,7 @@ Rome wasn't built in a day. DeciMojo is currently under active development. For
 - Proper implementation of traits (Absable, Comparable, Floatable, Roundable, etc).
 - **BigInt and BigUInt** implementations with complete arithmetic operations, proper division semantics (floor and truncate), and support for arbitrary-precision calculations.
 
-### Make it Fast ⚡ (SIGNIFICANT PROGRESS)
+### Make it Fast ⚡ (SIGNIFICANT PROGRESS) <!-- omit in toc -->
 
 DeciMojo delivers exceptional performance compared to Python's `decimal` module while maintaining precise calculations. This performance difference stems from fundamental design choices:
 
@@ -202,7 +212,7 @@ This architectural difference explains our benchmarking results:
 
 Regular benchmarks against Python's `decimal` module are available in the `bench/` folder, documenting both the performance advantages and the few specific operations where different approaches are needed.
 
-### Future Extensions 🚀 (PLANNED)
+### Future Extensions 🚀 (PLANNED) <!-- omit in toc -->
 
 - **BigDecimal**: 🔄 **IN PROGRESS** - Arbitrary-precision decimal type with configurable precision[^arbitrary].
 - **BigComplex**: 📝 **PLANNED** - Arbitrary-precision complex number type built on BigDecimal.

benches/bigdecimal/bench.mojo

@@ -3,6 +3,7 @@ from bench_bigdecimal_subtract import main as bench_sub
 from bench_bigdecimal_multiply import main as bench_multiply
 from bench_bigdecimal_divide import main as bench_divide
 from bench_bigdecimal_sqrt import main as bench_sqrt
+from bench_bigdecimal_exp import main as bench_exp
 from bench_bigdecimal_scale_up_by_power_of_10 import main as bench_scale_up
 
 
@@ -17,6 +18,7 @@ sub:         Subtract
 mul:         Multiply
 div:         Divide (true divide)
 sqrt:        Square root
+exp:         Exponential
 all:         Run all benchmarks
 q:           Exit
 =========================================
@@ -35,12 +37,15 @@ scaleup:     Scale up by power of 10
         bench_divide()
     elif command == "sqrt":
         bench_sqrt()
+    elif command == "exp":
+        bench_exp()
     elif command == "all":
         bench_add()
         bench_sub()
         bench_multiply()
         bench_divide()
         bench_sqrt()
+        bench_exp()
     elif command == "q":
         return
     elif command == "scaleup":