Quotation Monad

[gusty]

This will implement #44

A quotation monad allow us to compose functions returning expressions.

At the end of the composition we get a single expression, internally containing markers where an evaluate function needs to run a sub-expression.

Since F# doesn't support the "eval" expression, we can't directly run this expressions in existing quotation evaluators, but we include an Expr.run function which takes any normal quotation evaluator and takes care of handling the internal marker. So, instead of doing evaluator expr we do Expr.run evaluator expr.