Feature-gate explicit unboxed closure method calls & manual impls,

detect UFCS drop and allow UFCS methods to have explicit type parameters.

Work towards #18875.

Since code could previously call the methods & implement the traits
manually, this is a

[breaking-change]

Closes #19586. Closes #19375.
Curious, any particular reason why we want these codes commented here? This file isn't *really* meant for any sort of read consumption, it's more of a manifest.