feat!: replace lazy_static with once_cell

[AaronFeickert]

Extended commitment generators are currently produced using the lazy_static macro. Unfortunately, lazy_static is no longer actively maintained. Other repositories in the ecosystem are moving to once_cell, which is actively maintained.

This PR migrates from lazy_static to once_cell. In the process, it cleans up the construction. Specifically, it moves from using word-based numbering (e.g. ONE, TWO) in hash inputs to numerals, which simplifies the design. It also adds a useful test that asserts the number of generators matches the size of ExtensionDegree to avoid panics.

Closes #67.

BREAKING CHANGE: Modifies the construction of commitment generators.

[hansieodendaal]

Nice and concise, a definite improvement. Just a couple of small comments below.

[AaronFeickert]

There are still some assumptions that this code makes about ExtensionDegree that I don't particularly like:

• The enum will always contain only incrementing numerical values; this holds now due to the auto-incrementing functionality, but could be inadvertently changed later
• The MINIMUM and MAXIMUM constants will always refer to the lowest and highest valid enum values, respectively
• The TOTAL constant (defined in terms of the other constants) will always represent the total number of valid enum values

However, these assumptions are carefully documented, should be easy to check by inspection, are reasonably tested, and I don't see a good way to remove them without overhauling the ExtensionDegree design.

[hansieodendaal]

ACK

[hansieodendaal]

ACK

[hansieodendaal]

ACK