Remove the pbyte_offset field.
Remove the ~offset argument to Memory.build.
Remove the offset arguments from ctypes_memcpy.
Remove the offset argument to ctypes_read.
Remove offset arguments from ctypes_read_pointer and ctypes_write_pointer.
Remove the offset argument from ctypes_write.
Remove the offset argument from Ctypes_bigarray.view and supporting stubs.
Remove offset argument from Memory_stubs.string_of_array.
Remove offset argument to ctypes_string_of_cstring.
Merge pull request #168 from yallop/remove-offsets

Remove offsets
Change funptr's [release_runtime_lock] argument to [runtime_lock].

The [funptr] view can be used in either direction.  When [funptr] is
used to call C from OCaml, [runtime_lock] determines whether the lock
is released.  When [funptr] is used to call OCaml from C,
[runtime_lock] determines whether the lock is acquired.
Optionally acquire the runtime lock in the callback handler.
A test for the [runtime_lock] argument to [funptr].