syntax::diagnostics: Color the ^~~~ in green for better visibility

Fixes #7164.
Fix compiling empty fmt! strings
Merge pull request #7230 from Blei/green-squiggle

syntax::diagnostics: Color the ^~~~ in green for better visibility
Merge pull request #7270 from thestinger/doc

accumulated doc pull requests from the queue
Merge pull request #7227 from alexcrichton/issue-7218

Fix compiling empty fmt! strings
iterator: add a size_hint default method

also adds an implementation for the vector iterators
iterator: add a FromIterator trait

This is able to take advantage of the lower bound from the size hint.
vec: remove BaseIter implementation

I removed the `static-method-test.rs` test because it was heavily based
on `BaseIter` and there are plenty of other more complex uses of static
methods anyway.