Update/anchor v0.29 [WIP - help needed]

[dankelleher]

This is a WIP PR to upgrade anchor-gen to use anchor 0.29.

Note, it also incorporates work done by @datben to update to 0.28.

It is WIP, because it has a few gaps: Specifically, handling the new IdlTypes: GenLengthArray, Generic, and DefinedWithTypeArgs.

Some of these, I suspect, will need more code-generation to work properly. I have added some placeholder code just to get things to compile, but there are no tests that verify them right now, so I doubt they work.

See e.g. here and here

TODO list:

• [ ] Tests, highlighting 0.28 and 0.29 anchor features
• [ ] Fix the code around the new IdlTypes
• [ ] Fix build issues with the whirlpool example
• [ ] Add another example, that uses a more "modern" idl

[macalinao]

Nice work so far! hope someone else from the community can help pick this up, or potentially we could release a version that just can't support all IDLs