closes #292

* if, try, while, for, comptime, defer are "greedy" with {} blocks,
  meaning if their bodies are blocks, then no suffix operator is allowed
  after the block. The {} block gets "built into" the containing statement,
  like the body of a switch statement.
* the Expression syntactic element is no longer "greedy" with {} blocks,
  meaning it's possible to have suffix operators after {} blocks without
  needing the {} block to be an rhs operand first.