mk: Get "make check" passing with --disable-rpath

This involves passing through LD_LIBRARY_PATH through more places, specifically
in the compiletest, run-make, and doctest runners.
test: 'ratcher' is not a word
mk: restore check-notidy.

tidy has some limitations (e.g. the "checked in binaries" check doesn't
and can't actually check git), and so it's useful to run tests without
running tidy occasionally.
Fix emacs indentation of multi-line match patterns

Aligns to the same column if the previous line ends in a single '|' (but
not a '||').
std: mark two helper functions #[inline].

`str::utf8_char_width` and `char::from_u32` are tiny, which means it's a
big performance hit to call them in a tight loop outside libstd.
Fixed invalid JavaScript

Signed-off-by: Daniel Fagnan <dnfagnan@gmail.com>
Emacs: stay at the correct position when indenting

When indenting a non-blank line, stay at the same cursor position
relative to the content after indenting.
Emacs: indent relative to enclosing block

This changes the indent to calculate positions relative to the enclosing
block (or braced/parenthesized expression), rather than by an absolute
nesting level within the whole file.  This allows things like this to
work:

    let x =
        match expr {
            Pattern => ...
        }

With the old method, only one level of nesting would be added within the
match braces, so "Pattern" would have ended up aligned with the match.

The other change is that multiple parens/braces on the same line only
increase the indent once.  This is a very common case for passing
closures/procs.  The absolute nesting method would do this:

    spawn(proc() {
            // Indented out two indent levels...
    })

whereas the code in this commit does this:

    spawn(proc() {
        // Indented out only one level...
    })
Fix typo in emacs ERT test
std: Remove some nonsense from old std::io docs

Most of this stuff is irrelevant implementation notes from last year.
This trims out the stuff that isn't appropriate for user-facing docs.
Register snapshots.
Make break and continue hygienic

Makes labelled loops hygiene by performing renaming of the labels
defined in e.g. `'x: loop { ... }` and then used in break and continue
statements within loop body so that they act hygienically when used with
macros.

Closes #12262.
green: remove ancient register-saving code.

@alexcrichton said he thought this was useless (and it's old logic: it's
been there since before the runtime was written into Rust).
flate: return CVec<u8> rather than copying into a new vector.

This trades an O(n) allocation + memcpy for a O(1) proc allocation (for
the destructor). Most users only need &[u8] anyway (all of the users in
the main repo), and so this offers large gains.
green,native,rustuv: Replace many pointer `transmute`'s with `as` or referencing.

These can all be written in a more controlled manner than with the
transmute hammer, leading to (hopefully) safer code.
native: be more const correct with the FFI calls.

These calls are mutating their argument and so it's bad behaviour to be
pretending that the values are immutable to rustc.
arena,std,serialize: remove some unnecessary transmutes.

`as`-able transmutes, duplication and manual slice decomposition are
silly.
Fix C function FFI example in the Rust cheatsheet
Transition to new `Hash`, removing IterBytes and std::to_bytes.
Closes #12386. Removed 'pub mod' doc-comments in std::io's mod.rs file. Added summary doc-comments to test.rs, util.rs and stdio.rs.