Add aliases to arithmetic expressions

[ekmartin]

I removed multispace matching from string_literal and integer_literal since it swallowed the whitespace needed in as_alias (and it also seemed reasonable to do this at a higher level than in the literal matchers itself, but let me know if you disagree).

It seemed like the only change needed to do so was to add whitespace matching to predicate instead, which it already had for some of the branches.

I'll send a separate PR to add alias support for literals as well.

[ms705]

LGTM 👍