Add BigNumber::parse() / parseNullable()

[BenMorel]

Fixes #115.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (a89bc96) to head (39057fd).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #116      +/-   ##
============================================
+ Coverage     99.05%   99.08%   +0.03%     
- Complexity      779      818      +39     
============================================
  Files            21       21              
  Lines          1901     1967      +66     
============================================
+ Hits           1883     1949      +66     
  Misses           18       18              

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