Document for-loop in tutorial section on loops
 Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.
Adds support for working with URL Paths

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.
Fix bug with zero-length filemaps and rename bytepos_to_local_charpos to bytepos_to_charpos.
Fix typo in test metric ratchet stdout output
libnum: Make RandBigInt trait public

Closes #12383.
rustc: remove some unnecessary `transmute`s.

These can all be done by implicit or explicit &T -> *T casts, which are
more restricted and so are safer.
Fix getting/setting huge env vars on windows

On windows, the GetEnvironmentVariable function will return the necessary buffer
size if the buffer provided was too small. This case previously fell through the
checks inside of fill_utf16_buf_and_decode, tripping an assertion in the `slice`
method.

This adds an extra case for when the return value is >= the buffer size, in
which case we assume the return value as the new buffer size and try again.

Closes #12376
debuginfo: Add support for simd types
debuginfo: Move test case from run-pass to debug-info directory so it does not interfere with RUSTFLAGS=-g
debuginfo: Fix a RUSTFLAGS incompatibility in test runner.
debuginfo: Re-enable test case that should have worked all along.
Tweak how preference factors into linkage

The new methodology can be found in the re-worded comment, but the gist of it is
that -C prefer-dynamic doesn't turn off static linkage. The error messages
should also be a little more sane now.

Closes #12133
rustc: support dumping the AST as JSON

This is mostly useful for working on rustc, when one is unfamiliar with the
AST a particular construct will produce. It's a -Z flag as it's very much for
debugging.

Closes #10485
auto merge of #11904 : nick29581/rust/0filemap, r=alexcrichton
auto merge of #12231 : wycats/rust/url_path_parse, r=alexcrichton

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.

In theory I could have made Url embed a Path, but that would be a
breaking change and I assume that Servo uses this API. I would be
happy to update the PR to embed Path in Url if that's what people
wanted.
Don't recurse twice into ExprParen in vtable checking.
Replaced method_map_entry with method_origin and cleaned up vtable checking a bit.
adjust to currently used style
Resolve the vtables for method calls to generic Drop impls with trait bounds.