Generalize Emitter to support the LLVM backend

Currently, the `Emitter` interface assumes that you're producing strings. This works for generating JavaScript and GLSL, but it won't work for LLVM, where we'll need the functions to produce `llvm.Value` and other such objects.

One way to get started might be to make `Emitter` a generic interface: i.e., `Emitter` would be the current configuration. Or, it might be less disruptive to make the current `Emitter` inherit from a new `BaseEmitter` that adds more generality.

Start by tracing the Emitter interface and its JavaScript and GLSL implementations to understand the current string-producing contract. Define how that contract can support the LLVM backend's llvm.Value and related objects while preserving existing emitters; done means the design is implemented without breaking JavaScript or GLSL generation.

Written by the indexing model from the issue text.