clarifies that the interpreter only deals with TSTs
addresses review comments for the initial prototype

Initial prototype: https://github.com/dedoz/scala-macros-interpreter/commit/c6ba108c684a69f000b573ba52ef8f6b1466e2b1.
Comments addressed in this commit:
  1) Runtime reflection replaced by `sym.source`
  2) Interpreter is now written in functional style
  3) Fleshed out interpreter's data structures
  4) Execution state now uses symbols to track bindings