* instead of `async(allocator) call()`, now it is
`async<allocator> call()`.
* Fixes syntax ambiguity when leaving off the allocator
* Fixes parse failure when call is a field access
This sets a precedent for using `<` to pass arguments
to a keyword. This will affect `enum`, `union`, and
`fn` (see #661)
LLVM 5.0.1, 6.0.0, and trunk crash when attempting to optimize coroutine code.
So, Zig does not support ReleaseFast or ReleaseSafe for coroutines yet.
Luckily, Clang users are running into the same crashes, so folks from the LLVM
community are working on fixes. If we're really lucky they'll be fixed in 6.0.1.
Otherwise we can hope for 7.0.0.
