LIR definition and Cranelift backend

[nbp]

This pull request is a work-in-progress to handle the LIR to Cranelift IR conversion. It adds a LIR representation #29 and adds a Cranelift backend #28 .

However it does not yet change the MIR to LIR mapping nor the serialization of the LIR. Thus, this pull request cannot yet be tested with a complete pipeline to handle the Brainfuck example. Tests for this pull requests are added as part of the test of the newly added codegen sub-project. These tests are creating LIR units which are then lowered into Cranelift IR, compiled and tested.