Parse scripts and transpile to Outlines graphs

[rlouf]

This PR introduces scripts, a friendly interface to Outlines based on Mako templates. The scripts are callable, and when they are called with the variables to substitute, Mako's abstract syntax tree is transpiled into an Outlines graph.

import outlines

t = outlines.script("Test ${variable}")(variable=3.14)
type(t)
# StringVariable

I also add tests for the compiler:

s = string()
o = script("This is a ${var}")(var=s)
out = compile([s], [o])
out("test")
# This is a test