add a primitive to read an s-expression

[rain-1]

We will need a primitive that either:

• (A) invokes rackets s-expression reader
• (B) calls on our own s-expr reader

that allows us to easily include s-expressions inside PEG grammars.

(A) would depend on #2

perhaps (B) is the better option though. Even though we have to "re"implement s-expr parsing, it would blend more seamlessly with the rest of the peg parsing system.