Add new memory access interface to expr.c

Add new memory access interface to expr.c

DWARF expression evaluator is currently using a few different
interfaces for memory access: write_memory_with_notification,
read_value_memory, read_memory.

They all seem incosistent, while some of them even need a struct
value typed argument to be present.

This patch is simplifying that interface by replacing it with two new
low level functions: read_from_memory and write_to_memory.

The advantage of this new interface is that it behaves in the same way
as the register access interface from the previous patch. Both of these
have the same error returning policy, which will be usefull for the
following patches.

	* dwarf2/expr.c (xfer_from_memory):  New function.
	(read_from_memory): New function.
	(write_to_memory): New function.
	(rw_pieced_value): Now calls the read_from_memory and
	write_to_memory functions.

Change-Id: Id1f4c9b8127da83dece543e68757ffb1de2afedd