fix unused function warnings in codegen/powerpc/assembler.cpp when building with clang
adjust package structure in codegen (vm -> avian::codegen and avian::codegen::lir)

The eventual intent with the lir namespace is to formalize some of
the important bits of Assembler interface, to be tested, debug-printed,
and potentially, serialized.

Also, group arguments to apply(...) in OperandInfos
factor out assert / abort / expect implementations
begin moving register allocator out of compiler.cpp
begin splitting up compiler.cpp
move site out of compiler.cpp
further split up compiler.cpp
move Reads out of compile.cpp
move CallEvent out of compiler.cpp
move ReturnEvent out of compiler.cpp
move MoveEvent out of compiler.cpp
move CombineEvent out of compiler.cpp
move TranslateEvent out of compiler.cpp
move OperationEvent out of compiler.cpp
move MemoryEvent out of compiler.cpp
move BranchEvent out of compiler.cpp
move JumpEvent out of compiler.cpp
move BoundsCheckEvent out of compiler.cpp
move FrameSiteEvent out of compiler.cpp