[PERF] Instantiations are getting too far

As the documentation lints got approved, Ty::intern is now the public enemy number one. Today I analyzed all the occurrences of instantiate* functions and all Ty::interns.

Here is a list of all those calls that exceed 65 while running our test suite. Not all of these can be optimized away, but I'm sure that the greatest culprits can be optimized greatly.

There were lots of lints that use the instantiation functions <50 times in the whole test suite, so I'm pretty sure that (almost) any function that uses them > e.g. 5k times can be heavily optimized away.

For Ty::intern directly, only manual_abs_diff.rs::is_sub_expr is directly optimizable (100% of that), but it "only" creates 131 new types.
For instantiations:

• dereference.rs for_defined_ty 2.25k (calls)
• impl_hash_with_borrow_str_and_bytes.rs.html - 2.66k ..
• only_used_in_recursion - 2.66k ..
• mixed_read_write_in_expression - 20.8k
• needless_borrows_for_generic_args - 580
• large_enum_variant - 500
• ty/mod.rs 2.15k
• from_over_into - 241
• needless_match - 322
• utils/src/lib.rs - 9099
• functions/results.rs - 9660
• functions/ref_option.rs - 10488
• mut_key.rs - 22.8k (@blyxyas)
• non_copy_const - 1001
• methods/mod.rs - 9361
• self_named_constructors - 929
• useless_asref - 191
• path_buf_push_overwrite.rs - 168
• map_clone.rs - 482
• case_sensitive_file_extension_comparisons - 68
• unnecessary_to_owned - 469
• manual_ok_or - 90
• get_first - 105
• implicit_clone 107
• iter_on_single_or_empty_collections - 79
• unit_return_expecting_ord - 90630 (@blyxyas)
• mut_reference.rs - 17.7k
• transmute_undefined_repr - 349
• needless_pass_by_ref_mut - 80
• use_self.rs - 964
• derive.rs - 2.66k
• rest_pat_in_fully_bound_struct - 89
• large_const_arrays - 589

Run the test suite and inspect the listed instantiate* call sites and Ty::intern usage, starting with the highest-count entries such as utils/src/lib.rs, functions/results.rs, functions/ref_option.rs, and methods/mod.rs. Identify the greatest unnecessary instantiation costs, optimize the applicable sites, and confirm the test suite still passes with reduced calls.