Changes the style guidelines regarding unit tests to recommend using a
sub-module named "tests" instead of "test" for unit tests as "test"
might clash with imports of libtest.

(And other kinds of items).

This commit removes all the old casting/generic traits from `std::num` that are
no longer in use by the standard library. This additionally removes the old
`strconv` module which has not seen much use in quite a long time. All generic
functionality has been supplanted with traits in the `num` crate and the
`strconv` module is supplanted with the [rust-strconv crate][rust-strconv].

[rust-strconv]: https://github.com/lifthrasiir/rust-strconv

This is a breaking change due to the removal of these deprecated crates, and the
alternative crates are listed above.

[breaking-change]

Replace Path::exists with stable metadata call.

PR #24242 added the ability to the compiler to directly give suggestions about
how to modify code to fix an error. The new errors look like this:

    foobar.rs:5:12: 5:25 error: expected a path on the left-hand side of `+`,
                                not `&'static Copy` [E0178]
    foobar.rs:5     let x: &'static Copy + 'static;
                           ^~~~~~~~~~~~~
    foobar.rs:5:12: 5:35 help: try adding parentheses (per RFC 438):
    foobar.rs:      let x: &'static (Copy + 'static);

    foobar.rs:2:13: 2:23 error: cast to unsized type: `&_` as `core::marker::Copy`
    foobar.rs:2     let x = &1 as Copy;
                            ^~~~~~~~~~
    foobar.rs:2:19: 2:23 help: try casting to a reference instead:
    foobar.rs:      let x = &1 as &Copy;

    foobar.rs:7:24: 7:25 error: expected expression, found `;`
    foobar.rs:7     let x = box (1 + 1);
                                       ^
    foobar.rs:7:13: 7:16 help: try using `box()` instead:
    foobar.rs:      let x = box() (1 + 1);

This also modifies compiletest to give the ability to directly test suggestions
given by error messages.

This patch
1. renames libunicode to librustc_unicode,
2. deprecates several pieces of libunicode (see below), and
3. removes references to deprecated functions from
   librustc_driver and libsyntax. This may change pretty-printed
   output from these modules in cases involving wide or combining
   characters used in filenames, identifiers, etc.

The following functions are marked deprecated:

1. char.width() and str.width():
   --> use unicode-width crate

2. str.graphemes() and str.grapheme_indices():
   --> use unicode-segmentation crate

3. str.nfd_chars(), str.nfkd_chars(), str.nfc_chars(), str.nfkc_chars(),
   char.compose(), char.decompose_canonical(), char.decompose_compatible(),
   char.canonical_combining_class():
   --> use unicode-normalization crate

This allows #[derive(...)]` to create more than one impl

It was an oversight that this was not done in the great int upheaval.

[breaking-change]

Implement `Debug`, `Display` and `Error` for `FatalError` and `ExplicitBug`

`s/([^\(\s]+\.)len\(\) [(?:!=)>] 0/!$1is_empty()/g`

`s/(?<!\{ self)(?<=\.)len\(\) == 0/is_empty()/g`

This is a little bit tricky, since with include_str!, we know that we
are including utf-8 content, so it's safe to store the source as a
String in a FileMap. We don't know that for include_bytes!, but I don't
think we actually need to track the contents anyways, so I'm passing "".

new_filemap does check for the zero length content, and it should be
reasonable, howeven I'm not sure if it would be better to pass None
instead of Some(Rc::new("")) as the src component of a FileMap.

Fixes bug #24348

This commit entirely removes the old I/O, path, and rand modules. All
functionality has been deprecated and unstable for quite some time now!

Fixes #22619