[Rust API] Expose functions to map between IL levels

[bdash]

What is the feature you'd like to have?
The Rust API does not currently expose any way to map from an LLIL instruction to the corresponding MLIL instruction(s) or vice versa. Similarly absent are functions for MLIL <-> HLIL.

This would provide the equivalents of:

MediumLevelILFunction::GetLowLevelILInstructionIndex
MediumLevelILFunction::GetLowLevelILExprIndex
MediumLevelILFunction::GetLowLevelILExprIndexes

MediumLevelILFunction::GetHighLevelILInstructionIndex
MediumLevelILFunction::GetHighLevelILExprIndex
MediumLevelILFunction::GetHighLevelILExprIndexes

for each of the IL levels.