Add a vector type to the type system

[onelson]

This is in support of vectorizing float() calls, #4603

Since we're going to be adding a new builtin to flux that will accept a vector of values and return a vector of floats (in this case), the parser and type system need to understand what a vector is.

For example, the signature we're adding might be like:

builtin _vecFloat : (v: vec[T]) => vec[float]