type parameter inference

[graydon]

Currently if you have a function f[T](T t) you have to call it with explicit
type parameters, such as f[int](10) or f[char]('a'). This is not generally
necessary given the restricted set of contexts in which we permit type
parameters; the type inference system should be able to infer parameters.

Implement this.

[pcwalton]

Add automatic parameter instantiation. Closed by a7eeeb5.