target/tcl: Add 'read_memory' and 'write_memory'

target/tcl: Add 'read_memory' and 'write_memory'

These functions are meant as replacement for 'mem2array' and
'array2mem'.

The main benefits of these new functions are:

 * They do not use Tcl arrays but lists which makes it easier
   to parse (generate) the data. See the Python Tcl RPC code
   in contrib as a negative example.

 * They do not operate on Tcl variables but instead return (accept)
   the Tcl list directly. This makes the C and Tcl code base
   smaller and cleaner.

 * The code is slightly more performant when reading / writing
   large amount of data. Tested with a simple Python Tcl RPC
   benchmark.

Change-Id: Ibd6ece3360c0d002abaadc37f078b10a8bb606f8
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/6307
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>