Optimize TextFormatToIRReader API.

Make TextFormatToIRReader to return maps instead of vectors. This allows
us to 'merge' two TextFormatToIRReaders more efficiently, which enables
more efficient and cleaner code for multi-threaded linking.

Test: tests/test.py; all tests pass.

Change-Id: I6dae0080af9f36216d30370d4f4936eedcda83d1
9 files changed