Fix expanding to short versions of constructs

[hcpl]

This PR fixes 2 issues:

• The documentation shows that if_chain expands let and if let to equivalent Rust expressions, but that's not the case on latest master --- both expand to match expressions. This can be surprising when using macro expansion tools (e.g. cargo expand) and the result doesn't look like what one would expect after reading docs.
  Special handling for some cases should make the result clearer for users and potentially reduce burden placed on the optimizer, even if by a very small margin.
• Block matchers (or any AST node matchers in general) and verbatim code interact poorly: after {} is matched with a block matcher, it can't be matched with literal {} anymore. When that happens in a certain recursion step, for all next recursion steps the macro match arm with {} becomes unreachable and the code gets expanded to if { ... } else { {} }.
  The fix is to not use block matchers in presence of verbatim matching code. Generic tt matchers should be used in this case.

Example that exhibits both problems:

if_chain! {
   if let Some(y) = x;
   if y.len() == 2;
   if let Some(z) = y.first();
   then {
       do_stuff_with(*z);
   }
}

Expansion before this PR:

match x {
    Some(y) => {
        if y.len() == 2 {
            match y.first() {
                Some(z) => {
                    do_stuff_with(*z);
                }
                _ => {}
            }
        } else {
            {}
        }
    }
    _ => {}
}

Expansion after this PR:

if let Some(y) = x {
    if y.len() == 2 {
        if let Some(z) = y.first() {
            do_stuff_with(*z);
        }
    }
}

I think if_chain should have macro expansion tests, but I don't know how to set up something like that (compiletest_rs seems to not support that case).

[hcpl]

Travis fails on Rust 1.11 because of the old behavior for tt matchers that has been fixed in Rust 1.12 (confirmed that locally).

Is it OK to bump to Rust 1.12?

[lambda-fairy]

Is it OK to bump to Rust 1.12?

Sure, go for it! (And Merry Christmas 🙂)

[hcpl]

Sure, go for it!

Done.

(And Merry Christmas 🙂)

Whoops, Merry Super Belated Christmas (sorry for not noticing earlier, hope you don't mind :-) )!

I like the way you've commented each branch -- thanks for doing this!

No problem!

[lambda-fairy]

Thanks!