WIP Upgrade surface syntax.

Done:

 - Parse parenthesized expressions as infix.

 - Allow fixity declarations for user-defined operators.

 - Assume user-defined operators are infix, left-associative, and have
   precedence 6.

 - Update examples and tests to use infix generally.

 - Parse a parenthesized operator as postfix.

 - Start making prelude non-special.

To do:

 - Make infix the default.

 - Enforce that precedence is from 1 to 9 or.

 - Enforce no duplicate/conflicting operator declarations.

 - Give all prefix and postfix operators the same precedence.

 - Add semicolon pseudo-operator.

 - Change "-> { x y ... }" syntax to "-> x y ...;".

 - Kill the backslash syntax and parse mixfix functions.
Finish making prelude non-special.