Consolidate SexpToString cases
Allow an expression itself to express if it is literal

This provides a minor performance gain, as it saves a type assertion in
Eval. This is a silly way to go about performance improvements.
Fortunately this also paves the way for my next idea, which is to make
everything an expression; saving the need for the costly interface
assertion that's left in Eval.
use the PTInt type to express integer values

This is still backed by an int, but it now satisfies expression.
convert float and bool to Expressions
convert remaining types to Expressions
Convert all instances of interface{} to Expression

This allows for the elimination of the silly type assersion in Eval.
This also makes eval itself quite simple; most of the work has been
moved out to the type-specific Evals
Merge pull request #20 from ComputerDruid/expression

Expressionizing all the things