Just strings and numbers for now

This class exposes two functions. `evalNode` take an AST node and evaluates it down to simplest terms. `eval` takes an array of AST nodes and calls evalNode on each of them. I don’t think we’re quite ready to be producing HTML yet so let’s not get ahead of ourselves.

Currently I'm relying on JavaScript's semantics
around truthiness. I think I'm going to retain
that ease of use (i.e. `(if list #t #f)` should
return true if the list has more than 0 elements,
etc) but I want to specify that behavior myself.

Just numbers and strings for now

I added upper and lowercase because that seemed
preferable to always casting every html function
name to lowercase.

Because I was assuming that every expression
started with a literal, calls to `each` were
getting wrapped by a functionCall node with no
functionName, which was subsequently ignored by
the compiler. This removes all of that uselessness

No variables or conditionals or anything yet, just
the basic ability to turn s-expressions into HTML.