gdb/python: Add new gdb.Value.bytes attribute

gdb/python: Add new gdb.Value.bytes attribute

Add a gdb.Value.bytes attribute.  This attribute contains the bytes of
the value (assuming the complete bytes of the value are available).

If the bytes of the gdb.Value are not available then accessing this
attribute raises an exception.

The bytes object returned from gdb.Value.bytes is cached within GDB so
that the same bytes object is returned each time.  The bytes object is
created on-demand though to reduce unnecessary work.

For some values we can of course obtain the same information by
reading inferior memory based on gdb.Value.address and
gdb.Value.type.sizeof, however, not every value is in memory, so we
don't always have an address.

The gdb.Value.bytes attribute will convert any value to a bytes
object, so long as the contents are available.  The value can be one
created purely in Python code, the value could be in a register,
or (of course) the value could be in memory.

The Value.bytes attribute can also be assigned too.  Assigning to this
attribute is similar to calling Value.assign, the value of the
underlying value is updated within the inferior.  The value assigned
to Value.bytes must be a buffer which contains exactly the correct
number of bytes (i.e. unlike value creation, we don't allow oversized
buffers).

To support this assignment like behaviour I've factored out the core
of valpy_assign.  I've also updated convert_buffer_and_type_to_value
so that it can (for my use case) check the exact buffer length.

The restrictions for when the Value.bytes can or cannot be written too
are exactly the same as for Value.assign.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=13267

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>