* Add tests for pointer write command `*`
Some test cases override `asm.arch` for bitness compatibility reasons.
* Refactor pointer write command `*` to use the API
* Allow use of more general exprs as value for pointer write command `*` ##shell
* Instead of detecting when to write a value, and write a hexpair
otherwise, do the inverse: detect when to write a hexpair, and write a
value otherwise. This allows more general value exprs such as
variables, derefs, ...
* To enable this, add an option to `rz_hex_str_is_valid` so it can
reject a 0x-prefixed string (because 0x-prefixed strings should be
handled as values, not hexpairs, by the pointer write command)
* Add a test case to ensure a non-0x-prefixed value arg, that's not a
valid hexpair, now gets eval'ed as an expr by the pointer write
command (where previously it would throw an error)
* Add test for pointer read command `*`
Test for `asm.bits=8` uses `asm.arch=6502` because `asm.bits=8` is
unsupported for the default arch.
* Fix pointer command `*` broken since move from `?` to `%` ##shell
* Refactor to use appropriate existing API in `num_callback`
