Implement BitArray and replace trie.Key

[weiihann]

This PR implements a new data structure BitArray to replace trie.Key. The reasons being:

1. Misleading representation
   trie.Key doesn't actually just represent a Trie key in our use case. But rather, it’s a special data type that we use to just represent a fixed size of bit array with the number of bits used specified.

2. Inefficient operations
   The current implementation of the bitwise operations is scattered all around the place. We implement ourselves for some, and for others, we rely on conversion. BitArray implements everything from scratch without unnecessary conversion.

Regarding BitArray:

• It uses a little-endian format to perform bitwise operations more efficiently. All available operations are multiple times faster than trie.Key
• It is alloc-free
• It supports all use cases for trie and proofs (e.g. derive path from two bit arrays, check if path matches, etc)
• Adds built-in support for conversion (e.g. felt to bit array, bytes to bit array, etc)

Credits: some implementation details are inspired by https://github.com/holiman/uint256

[codecov]

Codecov Report

Attention: Patch coverage is 91.79894% with 31 lines in your changes missing coverage. Please review.

Project coverage is 74.83%. Comparing base (4ba933f) to head (fa237e8).

Files with missing lines	Patch %	Lines
core/trie/bitarray.go	92.85%	16 Missing and 2 partials ⚠️
core/trie/proof.go	87.80%	3 Missing and 2 partials ⚠️
core/trie/trie.go	90.24%	2 Missing and 2 partials ⚠️
core/trie/node.go	84.21%	1 Missing and 2 partials ⚠️
migration/migration.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2322      +/-   ##
==========================================
+ Coverage   74.42%   74.83%   +0.41%     
==========================================
  Files         110      110              
  Lines       11771    11901     +130     
==========================================
+ Hits         8760     8906     +146     
+ Misses       2329     2322       -7     
+ Partials      682      673       -9     

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[rodrigo-pino]

Just some comments. Let. me know what you think. Thanks for detailing the implementation!