Allow multiple inherent impls on primitive types

[alexcrichton]

In the standard library it's pretty unfortunate that we have to redefine all methods on string, slice, characters, and floating point types in libraries outside of libcore with an inherent methods. It's also unfortunate to have all the extension traits in libcore.

Let's just bite the bullet and allow multiple inherent impls on this type. It will...

• Reduce duplication (thereby preventing drift between API surface area)
• Remove unstable traits
• Perhaps be simpler in terms of resolve

We'll have to be sure to update rustdoc, however, to provide one unified view to the API surface area rather than multiple impl blocks.

cc @rust-lang/libs

[aturon]

cc @rust-lang/lang

[czipperz]

It appears this has been put on the back burner. Does anyone plan to implement this?

[Mark-Simulacrum]

We seem to mostly permit this today via some lang items (e.g.,

rust/src/libstd/f32.rs

Lines 28 to 29 in d0677b9

	#[lang = "f32_runtime"]
	impl f32 {

and

rust/src/libcore/num/f32.rs

Lines 147 to 149 in d0677b9

	#[lang = "f32"]
	#[cfg(not(test))]
	impl f32 {

) -- closing as such. If I've misinterpreted then we can of course reopen.