Port memory buffer from gdb-ui.el

Port memory buffer from gdb-ui.el
(gdb-memory-address): New variable which holds top address of
memory page shown in memory buffer
(gdb-memory-repeat-count, gdb-memory-format, gdb-memory-unit): New
customization variables.
New functions:
(gdb-display-memory-buffer, gdb-frame-memory-buffer): Functions to
display the memory buffer.
(gdb-memory-set-address, gdb-memory-set-repeat-count): Set memory
buffer display parameters.
(def-gdb-memory-format, gdb-memory-format-binary)
(gdb-memory-format-octal, gdb-memory-format-unsigned)
(gdb-memory-format-signed, gdb-memory-format-hexadecimal):
Functions for setting memory buffer format.
(gdb-memory-unit-word, gdb-memory-unit-halfword)
(gdb-memory-unit-giant, gdb-memory-unit-byte): Functions to set
unit size used in memory buffer.
(gdb-memory-show-next-page, gdb-memory-show-previous-page): Switch
to next/previous page of memory buffer.