zig fmt: preserve comments in front of test blocks

* refactor std.zig.parser
 * fix compiler crashing for some compile errors
 * take advantage of @field in std.zig.ast
 * move ast.NodeFoo to ast.Node.Foo
 * comment preservation is more explicit

See #911
6e57243a79Andrew Kelley committed on 4/20/2018, 6:15:09 AMparentcc35f08
3 files changedLine totals unavailable