fix: allow lowercase hex in Uint64

[natenichols]

High Level Overview of Change

Modifies HEX_REGEX to accept lowercase hex. fixes #132

Context of Change

Rippled api now encodes some UInt64 values as lowercase hex. Modifying this regex allows the binary codec to encode Uint64s that are lowercase hex.

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release