Feat improve bech32 perf

[remybar]

Related to https://github.com/keep-starknet-strange/joyboy/issues/74.

This PR uses Cairo 2.7.0 so it depends on the PR https://github.com/keep-starknet-strange/joyboy/pull/296 (which is blocked until snfoundry has been updated to work with Cairo 2.7.0).

Proposed optimizations:

• use a const fixed-size array for alphabet,
• direct conversion from ByteArray to 5-bits chunks,
• use for in loop in encode,
• avoid creating temporary arrays,
• hardcode generator loop in polymod,
• avoid using shr in polymod,
• call polymod directly, without intermediate arrays.

Note that all modifications have been done in bech32.cairo. The other modifications come from the PR https://github.com/keep-starknet-strange/joyboy/pull/296.