CorePPL→RootPPL compiler changes

[dlunde]

Daniel: CorePPL-to-RootPPL compiler additions (lower priority, but would be nice to have).

• Lambda lifting (now available in main Miking thanks to Lars)
• Passing functions as arguments (compile to function pointers). Current static analysis for finding resample-free functions becomes more involved, we then need 0-CFA?.
• Add check for well-formedness (early, before C translation probably)
• Allow for allocating pointer data types in PSTATE (i.e., in stochastic code). This can easily be done with malloc. Doing it by allocating directly on the PSTATE stack is more difficult, since it then involves relative pointers (PSTATE moves around in memory due to resampling). This is difficult to combine with absolute pointers without increasing overhead by adding runtime checks.
• Add support for mutable tensors at top level
• Ensure no warnings are omitted from GCC or Clang for RootPPL code generated by the CorePPL compiler.