Add Foundation.String.Show

[ndmitchell]

For every function in Foundation.String.Read it would be useful to have the equivalent show function. As for benchmarking, double-conversion is super fast but has a binding to a C++ library, which is awful and causes runtime dll errors on Windows. There is also floatshow and double-extra which are used, but not in Stackage.

[vincenthz]

this is somewhat stale on a string builder (I have local code for it, but not finished)

[ndmitchell]

A string builder was a related request which did come up (a list and append at the end is a serviceable alternative for now).