Rollup merge of rust-lang#77808 - Nicholas-Baron:fn_ctxt_impl, r=matthewjasper

Moved the main `impl` for FnCtxt to its own file.

Resolves rust-lang#77085 without breaking the API of the `FnCtxt` struct.

This is a solution to the file length being over 3000 (see issue rust-lang#60302).

The other solution to the file length is
1. to change the API of this struct by
2. encapulating certain fields of the struct into other structs.

Author: Dylan-DPC