edify: Some clean-ups to libedify.

- Remove dead declarations in expr.h: SetError(), GetError(),
  ClearError().
- Remove the declaration of Build() out of expr.h.
- Use std::unordered_map to implement RegisterFunction() and
  FindFunction(); kill FinishRegistration().
- Add a testcase for calling unknown functions.

Test: mmma bootable/recovery; recovery_component_test passes.
Change-Id: I9af6825ae677f92b22d716a4a5682f58522af03b