exp/types: Replace String method with TypeString function

exp/types: Replace String method with TypeString function

This is more in sync with the rest of the package;
for instance, we have functions (not methods) to
deref or find the underlying type of a Type.

In the process use a single bytes.Buffer to create
the string representation for a type rather than
the (occasional) string concatenation.

R=r
CC=golang-dev
https://golang.org/cl/6458057