Subdirectory "emitter"

Subdirectory "emitter"

Summary:
Move `emit_*.rs` along with a number of relevant files out of `hphp/hack/src/hhbc/hhbc_by_ref` into `hphp/hack/src/hhbc/emitter` and update dependencies
```
$ ls ~/fbsource/fbcode/hphp/hack/src/hhbc/emitter
cargo              emit_expression.rs        emit_memoize_method.rs  emit_property.rs       emit_typedef.rs    instruction_sequence.rs  reified_generics_helpers.rs
emit_adata.rs      emit_fatal.rs             emit_method.rs          emit_record_def.rs     emit_type_hint.rs  iterator.rs              statement_state.rs
emit_attribute.rs  emit_file_attributes.rs   emit_native_opcode.rs   emit_statement.rs      emit_xhp.rs        jump_targets.rs          symbol_refs_state.rs
emit_body.rs       emit_function.rs          emit_param.rs           emit_symbol_refs.rs    env.rs             label_rewriter.rs        TARGETS
emit_class.rs      emit_memoize_function.rs  emit_pos.rs             emitter.rs             generator.rs       label.rs                 try_finally_rewriter.rs
emit_constant.rs   emit_memoize_helpers.rs   emit_program.rs         emit_type_constant.rs  global_state.rs    local.rs                 typed_value.rs
```

Reviewed By: shayne-fletcher

Differential Revision: D33099378

fbshipit-source-id: 5082f2729476eaf6bdc49a58e8d7908f9f3d6140