Frozen ThreadState

[DoeringChristian]

This PR implements the Freezing feature in drjit-core (depends on #106).

It makes the following changes:

• Adds the freezing api to jit.h, including the following functions: jit_freeze_start, jit_freeze_stop, jit_freeze_replay, jit_freeze_dry_run, jit_freeze_pause, jit_freeze_resume, jit_freeze_abort and jit_freeze_destroy as well as the Recording struct.
• Adds the implementation of the RecordThreadState, a wrapper around either the CUDA or LLVM ThreadState for freezing them.

[DoeringChristian]

@merlinND, could you please do a preliminary pass over this PR, to catch some obvious mistakes of mine. Note, this also includes the commit from #106, which I'll remove when that gets merged.

[merlinND]

Review part 1. It's mostly about documentation / comments / docstrings to make everything super clear.

It's great that you already created unit tests at the drjit-core level!

I still have to look at the big record_ts.* files.

[merlinND]

Review part 2

[merlinND]

Review part 3.
It's a partial review of record_ts.cpp, but unfortunately I won't have time to review the rest of the file, there's too much code.

[wjakob]

Hi Christian,

this is very impressive work! Here is a first batch of feedback, much of it about the structure and comments.

[DoeringChristian]

Squashed everything into 7 commits. The first two are the same as in #106.