In addition to implicit lambdas, also add implicit partial application for operator functions

Broken off of #36. From that issue:

@evhub: In addition to implicit lambdas as was implemented here, I think it would also be a good idea to add implicit operator functions using the (? + 5) syntax, which would be equivalent to (+)$(?, 5), since I think the former is a lot easier to read. Reopening this issue to add that syntax. @fredcallaway: I don’t love the idea of a special syntax restricted to operator expressions. Can you come up with a syntax that works cleanly for both (? + 5) and (foo(?) + 5)? For example, something like option 3 above. @evhub: @fredcallaway The syntax (-> _ + 5) or (-> foo(_) + 5) was already added and handles those use cases. The rationale for (? + 5) is that it would generate a partially-applied operator function instead of a lambda, which is significantly better for pickling, casting to a str, profiling, etc. @evhub: Maybe the right solution is just to detect implicit lambdas of the form (-> _ + 5) and automatically compile them to partially-applied operator functions, but that seems too implicit and weird to me, so I’m wary to do it.

[yggdr]

I would prefer some variation of Elixir’s syntax: &(&1 + &2). Maybe substituting $ for & since that’s already associated with partial functions.

[evhub]

New proposed syntax is to use a single dot in place of the missing argument. The implicit partial must be contained in parentheses, just like operator functions, and the implicit argument can only be a single atom. Examples of valid syntax:

(.-1)
(a+.)
(f() .. .)