Fill in example code for HashMap.

Add an example showing how to use the map with a custom type. Fill in
examples for methods in the hashmap file without ones.

Also move pop_equiv next to related public methods, to not create a
duplicate trait implementation in the docs.
Remove explicit rust code specifier. Unhide use HashMap.
Move contructors to the top of PriorityQueue.
Document PriorityQueue.
auto merge of #15921 : dotdash/rust/match_lifetimes, r=pcwalton

The allocas used in match expression currently don't get good lifetime
markers, in fact they only get lifetime start markers, because their
lifetimes don't match to cleanup scopes.

While the bindings themselves are bog standard and just need a matching
pair of start and end markers, they might need them twice, once for a
guard clause and once for the match body.

The __llmatch alloca OTOH needs a single lifetime start marker, but
when there's a guard clause, it needs two end markers, because its
lifetime ends either when the guard doesn't match or after the match
body.

With these intrinsics in place, LLVM can now, for example, optimize
code like this:

````rust
enum E {
  A1(int),
  A2(int),
  A3(int),
  A4(int),
}

pub fn variants(x: E) {
  match x {
    A1(m) => bar(&m),
    A2(m) => bar(&m),
    A3(m) => bar(&m),
    A4(m) => bar(&m),
  }
}
````

To a single call to bar, using only a single stack slot. It still fails
to eliminate some of checks.

````gas
.Ltmp5:
	.cfi_def_cfa_offset 16
	movb	(%rdi), %al
	testb	%al, %al
	je	.LBB3_5
	movzbl	%al, %eax
	cmpl	$1, %eax
	je	.LBB3_5
	cmpl	$2, %eax
.LBB3_5:
	movq	8(%rdi), %rax
	movq	%rax, (%rsp)
	leaq	(%rsp), %rdi
	callq	_ZN3bar20hcb7a0d8be8e17e37daaE@PLT
	popq	%rax
	retq
````

Refs #15665
Documentation examples for LruCache.
auto merge of #15856 : treeman/rust/doc-priorityqueue, r=huonw

Add examples to methods.
Cleanup LruCache doc.
Improved the tags impl regex
auto merge of #15922 : poiru/rust/remove-whitespace-mk-backslash, r=brson

The alignment of the line continuation backslashes is rather inconsistent. These commits solve that by removing the extra whitespace and adding a space where there previously was none. An alternative solution would be to fix the alignment.
Cleanup HashMap documentation.

Link to mentioned methods. Use `# Failure` tags to describe failure.
Make `pop_equiv`, `find_equiv` and `get_copy` standalone.
Remove OwnedStr trait

This trait was only implemented by `String`. It provided the methods
`into_bytes` and `append`, both of which **are already implemented as normal
methods** of `String` (not as trait methods). This change improves the
consistency of strings.

This shouldn't break any code, except if somebody has implemented
`OwnedStr` for a user-defined type.
Deprecated `str::raw::from_c_str`

Use `string::raw::from_buf` instead

[breaking-change]
Deprecated `str::raw::from_utf8_owned`

Replaced by `string::raw::from_utf8`

[breaking-change]
Deprecated `str::raw::from_byte`

Use `string:raw::from_utf8` instead

[breaking-change]
Deprecated `str::raw::from_buf_len`

Replaced by `string::raw::from_buf_len`

[breaking-change]
Deprecated `String::from_raw_parts`

Replaced by `string::raw::from_parts`

[breaking-change]
Fix travis errors
Add `string::raw::from_buf`
Fix deprecation warning in deque.rs