Fix union, clean up expressions, add expected tokens

This works only on my patched version of Happy.

  * 'union[1] + 3' parses as a statement
  * expressions grammar is factored to avoid repetition
  * failed parses include expected tokens in the error message

This is going on a seperate branch because it won't pass CI. That
aside, ALL tests pass.

Author: harpocrates

src/Language/Rust/Parser/Internal.y

@@ -963,7 +963,6 @@ postfix_blockexpr(lhs) :: { Expr Span }
     }
 
 
-
 -- Then, we instantiate this general production into the following families of rules:
 --
 --   ['expr']               Most general class of expressions, no restrictions