Use map indexed by TermIndex instead of Map Term.

[brianhuffman]

This avoids relying heavily on the slow Ord instance for type Term,
which can become a significant slowdown when bindSAWTerm is called
many times.

[brianhuffman]

On the saw-script ecdsa-crucible.saw proof, this change cuts the total runtime from 5:47 down to about 5 minutes on my machine (about a 13% speedup).

[brianhuffman]

Some tests on macos are failing with messages like this:

      strlen_test:   FAIL (0.32s)
        Test output was different from 'test-data/golden/strlen_test.good'. It was:
        [Crux] Invalid record size: 22
        Expected size between 14 and 19
        Are you sure you're using a supported version of LLVM/Clang?
        Check here: https://github.com/GaloisInc/llvm-pretty-bc-parser
        
        from:
        	METADATA_COMPILE_UNIT
        	METADATA_BLOCK
        	METADATA_BLOCK_ID
        	value symbol table
        	MODULE_BLOCK
        	Bitstream
        
        *** Crux failed with exception: "ExitFailure 1"

@robdockins, are these ignorable failures?

[robdockins]

Yes, you can ignore these. This is due to a recent clang update on the MacOS build systems. #577