syntax: Tweak parsing lifetime bounds on closures

[alexcrichton]

In summary these are some example transitions this change makes:

'a ||       => ||: 'a
proc:Send() => proc():Send

The intended syntax for closures is to put the lifetime bound not at the front
but rather in the list of bounds. Currently there is no official support in the
AST for bounds that are not 'static, so this case is currently specially handled
in the parser to desugar to what the AST is expecting. Additionally, this moves
the bounds on procedures to the correct position, which is after the argument
list.

The current grammar for closures and procedures is:

procedure := 'proc' [ '<' lifetime-list '>' ] '(' arg-list ')'
                    [ ':' bound-list ] [ '->' type ]
closure := [ 'unsafe' ] ['<' lifetime-list '>' ] '|' arg-list '|'
                    [ ':' bound-list ] [ '->' type ]
lifetime-list := lifetime | lifetime ',' lifetime-list
arg-list := ident ':' type | ident ':' type ',' arg-list
bound-list := bound | bound '+' bound-list
bound := path | lifetime

This does not currently handle the << ambiguity in Option<<'a>||>, I am
deferring that to a later patch. Additionally, this removes the support for the
obsolete syntaxes of ~fn and &fn.

Closes #10553
Closes #10767
Closes #11209
Closes #11210
Closes #11211

[alexcrichton]

Note that I consider this closing all of #11209, #11210, and #11211. I'd like to open a separate bug for the << issue, which I will nominate but not not believe should block 1.0

[lilyball]

What is the ['<' lifetime-list '>'] in closures? I've never seen <'a> || before.

[alexcrichton]

I would recommend taking a peek at the test I added. The lifetime list is meant to introduce lifetimes that the closure itself can use to connect parameters to one another:

<'a> |foo: &'a Foo| -> &'a int