Allow escodegen sourcemap generation

This PR allows the instrumenter to return a source map along with the instrumented code, which can be useful if you want to go backwards from instrumented to original code. If the user configures the code generation options to generate a source map, it's returned in an object, along with the generated code, by calls to `isntrumentASTSync`. Otherwise, `instrumentASTSync` behaves as before, returning the generated code in a string. I've also included a couple basic tests.