lang: Shorten invariant lifetimes during Context creation

[acheroncrypto]

Problem

As mentioned in #3340 (comment), the helper function __shrink_lifetime actually extends the mutable borrow lifetime instead of shrinking the inner lifetime:

https://github.com/solana-foundation/anchor/blob/62865c636aecc6974fc9cfebfc6cf08ca4f0bb72/lang/syn/src/codegen/program/handlers.rs#L130-L132

This is not the desired behavior, and it also conflicts with its safety comments:

https://github.com/solana-foundation/anchor/blob/62865c636aecc6974fc9cfebfc6cf08ca4f0bb72/lang/syn/src/codegen/program/handlers.rs#L138-L142

Summary of changes

Shorten invariant lifetimes rather than extending the mutable borrow lifetime during Context creation.

[vercel]

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[jamie-osec]

Just bumping on this, are you able to fix the compile errors?

[acheroncrypto]

There seems to be only one compile error coming from that cursed generics accounts struct. It should be fixable. I'll take a look later this week.