Improve NativePointer#toString and add toJSON

If no radix is specified for toString() we should make the string human-
readable by defaulting to base 16 and adding a "0x" prefix.
Expose NativePointer C++ API to other script modules
Fix callback-argument declaration in gum_script_callbacks_get[_opt]
Generate a Stalker call summary if onCallSummary callback is provided

The summary is an object mapping call target to frequency, which is useful
for applications that want to display a heat map without potentially
transferring huge amounts of data across process boundaries.