Design and implement the AIR linear IR for the JIT compiler

[alexrp]

Architecture-Level IR (AIR) will be used by the JIT compiler only. As its name implies, it will feature architecture-specific operations to the extent that this makes sense for a given architecture. Unlike LIR, untyped pointers do not exist at this stage; we are only targeting 64-bit systems with the JIT compiler, so only 64-bit integers and 64-bit floats exist. Register allocation and likely some peephole optimizations will be performed on AIR before machine code generation.

Depends on:

• #60