[decimal] Implement ln for BigDecimal (#76)

This pull request introduces several enhancements and fixes to the
`bigdecimal` module, including the addition of a new natural logarithm
function, renaming and refactoring of existing methods for clarity, and
updates to dependencies and test cases.

### New Features:
* Added a natural logarithm function to the `bigdecimal` module (`ln`).
[[1]](diffhunk://#diff-be2f9b2702fd15952546e17fe94a64c22902a0e4678bfd9cb925269647923db4R545-R549)
[[2]](diffhunk://#diff-8edc842002d08323642db57f775ff7f626be068d55cb55bbe637e215f68ed8bcR349-R580)

### Method Renaming and Refactoring:
* Renamed `true_divide_fast` to `true_divide_inexact` and updated its
implementation to use `number_of_significant_digits` instead of
`minimum_precision`.
[[1]](diffhunk://#diff-f79534f4e7fdd891932ce9d015c50bd3c8a72c4a1689f0cb55524490ffc0458dL334-L359)
[[2]](diffhunk://#diff-f79534f4e7fdd891932ce9d015c50bd3c8a72c4a1689f0cb55524490ffc0458dL370-R363)
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### Dependency Updates:
* Added `mpmath` as a new dependency in `mojoproject.toml`.

### Error Message Improvements:
* Enhanced error messages in `BigUInt` struct methods for better
clarity.
[[1]](diffhunk://#diff-f9432b9b2671643af91201f9e3f011551a3d3b0e6d7b256d0d4569f5ae59848aL230-R230)
[[2]](diffhunk://#diff-f9432b9b2671643af91201f9e3f011551a3d3b0e6d7b256d0d4569f5ae59848aL280-R287)

### Test Case Updates:
* Updated test cases to include tests for the natural logarithm
function.

Author: forfudan

benches/bigdecimal/bench.mojo

@@ -4,6 +4,7 @@ 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_ln import main as bench_ln
 from bench_bigdecimal_scale_up_by_power_of_10 import main as bench_scale_up
 
 
@@ -19,6 +20,7 @@ mul:         Multiply
 div:         Divide (true divide)
 sqrt:        Square root
 exp:         Exponential
+ln:          Natural logarithm
 all:         Run all benchmarks
 q:           Exit
 =========================================
@@ -39,13 +41,16 @@ scaleup:     Scale up by power of 10
         bench_sqrt()
     elif command == "exp":
         bench_exp()
+    elif command == "ln":
+        bench_ln()
     elif command == "all":
         bench_add()
         bench_sub()
         bench_multiply()
         bench_divide()
         bench_sqrt()
         bench_exp()
+        bench_ln()
     elif command == "q":
         return
     elif command == "scaleup":