[integer] Refine arithmetic functions of BigUInt (#98)

Refine the arithmetic functions for `BigUInt`

1. Add `subtract_inplace()`
2. Optimize the cases when the second argument is one-word long.
3. Refine the estimation of quotient for division using 3-by-2 division.
4. Update the function `is_one()` and `is_two`.
5. Remove unused function `floor_divide_partition()`.

Author: forfudan

README.md

@@ -21,7 +21,7 @@ DeciMojo provides an arbitrary-precision decimal and integer mathematics library
 The core types are:
 
 - A base-10 arbitrary-precision signed integer type (`BigInt`) and a base-10 arbitrary-precision unsigned integer type (`BigUInt`) supporting unlimited digits[^integer]. It features comprehensive arithmetic operations, comparison functions, and supports extremely large integer calculations efficiently.
-- An arbitrary-precision decimal implementation `BigDecimal` allowing for calculations with unlimited digits and decimal places[^arbitrary].
+- An arbitrary-precision decimal implementation `BigDecimal` allowing for calculations with unlimited digits and decimal places[^arbitrary]. It provides a complete set of arithmetic operations, comparisons, and mathematical functions like logarithms, exponentiation, roots, trigonometric functions, etc. It also supports rounding modes and conversions to/from built-in types.
 - A 128-bit fixed-point decimal implementation (`Decimal`) supporting up to 29 significant digits with a maximum of 28 decimal places[^fixed]. It features a complete set of mathematical functions including logarithms, exponentiation, roots, etc.
 
 This repository includes [TOMLMojo](https://github.com/forfudan/decimojo/tree/main/src/tomlmojo), a lightweight TOML parser in pure Mojo. It parses configuration files and test data, supporting basic types, arrays, and nested tables. While created for DeciMojo's testing framework, it offers general-purpose structured data parsing with a clean, simple API.

benches/biguint/bench.mojo

@@ -12,7 +12,7 @@ This is the BigUInt Benchmarks
 =========================================
 add:         Add
 mul:         Multiply
-trunc:       Truncate divide (//)
+div:         Truncate divide (//)
 fromstr:     From string
 all:         Run all benchmarks
 q:           Exit
@@ -24,7 +24,7 @@ q:           Exit
         bench_add()
     elif command == "mul":
         bench_multiply()
-    elif command == "trunc":
+    elif command == "div":
         bench_truncate_divide()
     elif command == "fromstr":
         bench_from_string()