Make sicmutils.expression.compile backend pluggable

[sritchie]

Right now the compile functions use a dictionary called compiled-fn-whitelist to associate symbols with functions. After simplification and common subexpression elimination, the compiler swaps these functions back in for their associated symbols and calls eval on the result.

There are a few alternatives a user might want to the current compiled-fn-whitelist:

• Swap the GENERIC functions back in, so you can perform a simplification but still stay in flexible, generic land
• super fast math on the JVM, via Geex or Fastmath

Geex looks like it might be the easiest to integrate.

[sritchie]

@littleredcomputer this is another upgrade we'll want to try eventually, I think...