token 2022: add alloc_and_serialize for fixed-len extensions

[buffalojoec]

This PR adds the alloc_and_serialize function for fixed-length extensions to Token2022.

Note: I decided to follow already pre-established patterns and rename the existing alloc_and_serialize function to alloc_and_serialize_variable_len_extension. The existing pattern has been to add some form of *variable_len* suffix on functions that deal with variable-length methods, and give the more concisely named functions to the fixed-length extensions.

Regardless, this function allows reallocation of an account only if the extension does not exist yet and requires more uninitialized bytes to write it in.

[buffalojoec]

Current dependencies on/for this PR:

• master
  • PR token 2022: add alloc_and_serialize for fixed-len extensions #5672 👈
    • PR token 2022: add InitializeGroup instruction from SPL Token Group interface #5601
      • PR token 2022: add UpdateGroupMaxSize instruction from SPL Token Group interface #5626
        • PR token 2022: add UpdateGroupAuthority instruction from SPL Token Group interface #5627
          • PR token 2022: add support for _reading_ repeating fixed-length extensions #5836
            • PR token 2022: add support for _writing_ repeating fixed-length extensions #5837
              • PR token 2022: add support for additional signers in test init functions #5838
                • PR token 2022: add GroupMemberPointer extension #5628
                  • PR token 2022: add alloc_and_serialize_allow_repeating #5839
                  • PR token 2022: add InitializeMember instruction from SPL Token Group interface #5629

This stack of pull requests is managed by Graphite.

[joncinque]

This is looking very close! Just a few nits, but the core is good. Could you add a test for the new function?

[buffalojoec]

Could you add a test for the new function?

Yep, I was mostly waiting to see if you were okay with changing this many bits of the TLV/extension module. Didn't want to make too many changes.

Will definitely add some tests!

[joncinque]

Just a few last little things, then this is good to go! The tests look great

[joncinque]

Thanks for integrating those changes!