PredictionMode rewrite.

Dropped reliance on FlexibleHashMap in favor of C++ standard
containers.  May need to update the hash/comparison code elsewhere
later on.

Reformatted using clang-format (google style — up to any style as long
as we have one).
Remove FlexibleHashMap

FlexibleHashMap no longer used by PredictionMode as it now uses
standard containers for its implementation.
Fix incomplete class error.

Putting the class header file first followed by the standard headers
and the project headers is a good pattern which works here.
RuleContext cleanup.

A few fixes here and there.
A few miscellaneous fixes.
Fixes
Clean up PredictionContextCache
Add misc namespace to IntegerList references.

Also reformats the file using clang-format.

(We’ll probably drop IntegerList in favor of std::vector<int> at some
point.)
ConsoleErrorListener cleanup.