feat: Watchpoints

[ndrewh]

We can instrument memory accesses, following the example of https://github.com/DynamoRIO/dynamorio/blob/master/api/samples/memtrace_simple.c.

An initial prototype can be slow, as long as the instrumentation is disabled by default.

I think several optimizations will eventually be necessary.

• We cannot afford a clean-call all the way out to Python for every memory access. At a minimum, filtering needs to take place in pyda_core -- ideally inline. Users likely only care about a range (or: several ranges) of memory addresses, and our instrumentation should check the address (using in-line instrumentation) so it only performs a clean-call at all if the access is within one of these regions.
• One potential optimization is to make watched accesses cause a fault (i.e. by changing page protections), and then handling the fault by selectively inserting instrumentation into the block where the fault occurred