Explore exporting buildType and buildSchema functions

[migueloller]

build is very useful and convenient in that it can produce any number of types. Sometimes it might be inconvenient to get a type map though.

If we export the internal buildSchema and buildType functions then the developer could potentially use these to build a single type (or schema) and expect a GraphQLType or GraphQLSchema as an output instead of a map of those 2 things.

Would love to hear people's thoughts on adding these to the public API.