Rather than letting the reduced tree become the new lookahead symbol,
and re-adding it to the stack via a subsequent shift action, just
add it to the stack as part of the reduce action. This is more in
line with the way LR is described traditionally.
Generated parsers no longer export a parser constructor function.
They now export an opaque Language object which can be set on
Documents directly. This way, the logic for constructing parsers
lives entirely in the runtime. The Languages are just structs which
have no load-time dependency on the runtime
