str: Add better tests for string slice's Chars iterator

Test using for ch s.chars() { black_box(ch) } to have a test that should
force the iterator to run its full decoding computations.
core::str: Implement Chars iterator using slice::Items

Re-use the vector iterator to implement the chars iterator.

The iterator uses our guarantee that the string contains valid UTF-8,
but its only unsafe code is transmuting the decoded u32 into char.
Add more tests for str Chars iterator

Test iterating (decoding) every codepoint.
sync: Ensure try_send() wakes up receivers

This branch of try_send() just forgot to wake up any receiver waiting for data.

Closes #15761
Add examples for DList methods rotate_forward, rotate_backward, append, prepend and insert_when
Fill in documentation for HashSet.

Example how to use the set with a custom type. Fill in examples for the missing methods.
Remove examples from trait implementations. Unhide imports.
Include Strings guide with the others.

Three small changes:

1. Re-organize headers in the Strings guide so they show up correctly.
2. build the strings guide with the other docs
3. include the strings guide in the list of guides
rustc: #[crate_name] and --crate-name must match

Part of the original discussions around the `--crate-name` flag brought up that
mass confusion can arise when the flag specifies a different name than is
contained in the crate.

The current primary use case of the `--crate-name` flag is through cargo and
not requiring a `#[crate_name]` attribute, but if the `#[crate_name]` attribute
is specified it will likely go awry when the two names deviate from one another.
This commit requires that if both are provided they both match to prevent this
confusion.
Assign more diagnostic codes
librustc: Emit enum variant constructor at callsite instead of via a call to a function.
librustc: Emit tuple struct constructor at callsite instead of via a call to a function.
librustc: Only emit constructor functions as necessary.
librustc: Set enum discriminant only after field translation.
Ignore one test.
Clarify str Chars iterator implementation

Thanks to comments from @huonw, clarify decoding details and use
statics for important constants for UTF-8 decoding. Convert some magic
numbers scattered in the same file to use the statics too.
Write multibyte case for str Chars iterator in-line

Thanks to comments from @alexcrichton, write the next/next_back function
bodies without nested functions in a more top-to-bottom flow style.

Also improve comment style and motivate the unsafe blocks with comments.
Add support for patterns referencing non-trivial statics

This is accomplished by rewriting static expressions into equivalent patterns.
This way, patterns referencing static variables can both participate
in exhaustiveness analysis as well as be compiled down into the appropriate
branch of the decision trees that match expressions are codegened to.

Fixes #6533.
Fixes #13626.
Fixes #13731.
Fixes #14576.
Fixes #15393.
rustc: Mix extra-filename in temp outputs

When invoking the compiler in parallel, the intermediate output of the object
files and bytecode can stomp over one another if two crates with the same name
are being compiled.

The output file is already being disambiguated with `-C extra-filename`, so this
commit alters the naming of the temporary files to also mix in the extra
filename to ensure that file names don't clash.
Improve documentation for rand::random

This is now linked to in the guide, so I want to make sure it's good. This
adds a bit more explanation, and brings usage in line with current good style.