reimplement mustache-async using bytecode

Convert mustache-async to byte code, and create an interpreter for it. Probably make sense to generate explicit opcodes for pushing and poping values from the stack.

In Lua there should be 'compile' and 'eval' functions available. The interface to Lua could be improved (instead of the this object, maybe have read(key) and object() functions (returning one field, or everything).

With the new defaults for templates functions, we can probably eliminate the Context AST node.