Move trait impls for primitives near trait definition

Closes #12925
auto merge of #14464 : Sawyer47/rust/issue-12925, r=alexcrichton

This is an attempt of fixing #12925.

This PR moves almost all trait implementations for primitive types ((), bool, char, i*, u*, f*) near trait definitions. Only Float trait implementations weren't moved because they heavily rely on constants defined in f32.rs and f64.rs.

Some trait implementations had cfg(not(test)) attribute. I suspect it's because of issue 2912.
Still, someone who knows the problem better should probably check this code.

Closes #12925