[debugger] Remove out param for thread_read_state

The size of data being read and written should always be known in advance by
both caller and callee.

Removes backward-compatibility definitions from debug.h since the API is
currently changing in a non-backwards-compatible-way and we need to update
callers anyway. There were many changes associated with removing some users of
these structs and defines.

Add thread_read_state and thread_write_state to the zx::thread class.

Change-Id: I6d7e428d337f2722c529b68074dff16356984d63

[debugger] Clarify thread state API

Firms up the contract for zx_read|write_thread_state and makes the state more
explicit for each platform.

The general register struct has been made for the current platform which
simplifies much fo the code.

The sizes are converted to size_t for consistency with other object calls.

Forwarding #defines and typedefs are included for downstream users until they
can be updated.

This will likely need to be revised somewhat again when we add more thread
states, which I plan to do in a followup.

Change-Id: I57e8e9979d073c8efdfaeb2683f9f83494d19abf

[ulib][inspector] Provide C API.

The patch is separated out to simplify the extraction of the library
from crashlogger. This is part 2 of the extraction.

Change-Id: Iec66b22b1de0bcb55970f2717865bec4c85bea43

[ulib][inspector] Make a library of crashlogger's backtracer

Change-Id: I7de0ac4d17bdaeb447a9eca00698cd4f91df9117