Remove redundant functions from libraries

[brson]

Methods in the libraries are mostly implemented as wrappers for standalone functions. This wasn't really an intentional design decision, but just an easy way to start using methods. We should keep the methods and get rid of the functions. Methods are more convenient for multiple reasons.

[catamorphism]

Sometimes it's convenient to have a function so you can write v.map(f) instead of v.map(|x| x.f())...

[catamorphism]

Ongoing cleanup, not critical for 0.6. De-milestoning

[pcwalton]

Nominated for backwards compatibility. Unified Method Syntax will allow a lot of this to be fixed.

[thestinger]

I think we all agree that these should be removed and methods be made callable as functions instead, so I'm removing the RFC tag.

[graydon]

see also #6974, #6045

[graydon]

accepted for backwards-compatible milestone

[huonw]

This has progressed: std::{vec, str, option, result} have had most of their free-standing functions removed (where relevant), and most new libraries are using methods as much as possible.

However, it's definitely not finished.

[alexcrichton]

I think it's safe to say that this is pretty much done at this point. There's pressure against this all on all new PRs, the stdlib is in good shape with respect to these freestanding functions, and it's been quite awhile since I've used a freestanding function myself.

There may still be remaining issues, but I think they can be addressed on a case-by-case basis, the reach of this bug has diminished by a fair amount.