InputCommon: Add a new ExpressionParser to replace the old hack language

This contains a new, hand-written expression parser to replace the old
hack language based on string munging. The new approach is a simple
AST-based evaluation approach, instead of the "list of operations"
infix-based hack that there was before.

The new language for configuration has support for parentheses, and
counts "!" as a unary operator instead of the binary "NOT OR" operator
it was before. A simple example:

  (X & Y) | !B

Explicit device references, and complex device names ("Right Y+") are
handled with backticks and colons:

  (`SDL/0/6 axis joystick:Right X+` & `DInput/0/Keyboard Mouse:A`)

The basic editor UI that inserts tokens has not been updated to reflect
the new language.

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