Minor fixes to `std::str` docs

This tweaks the `std::str` docs to compensate for the recent shift from `~str`
to `String`.
syntax: Add a source field to `Local` for tracking if it comes from `let`s or `for`s.
syntax: desugar a `for` loop to a let binding to get better error
messages when the pattern is refutable.

This means the compiler points directly to the pattern and said that the
problem is the pattern being refutable (rather than just saying that
some value isn't covered in the `match` as it did previously).

Fixes #14390.
rustc: provide more precise information about refutable patterns.

The compiler now points exactly which part(s) of a pattern are
refutable, rather than just highlighting the whole pattern.
auto merge of #14435 : P1start/rust/str-docs-fix, r=sfackler

This tweaks the `std::str` docs to compensate for the recent shift from `~str` to `String`.
auto merge of #14444 : huonw/rust/nice-for-errors, r=alexcrichton

Change `for` desugaring & make refutable pattern errors more precise

This changes for to desugar to the `let`-based pattern match as described in #14390, and adjusts the compiler to use this information for error messages that even mention that it's in a `for` loop.

Also, it makes the compiler record the exact positions of refutable parts of a pattern, to point to exactly them in error messages.