This basically introduces the MICROPY_MACHINE_MEM_GET_READ_ADDR
and MICROPY_MACHINE_MEM_GET_WRITE_ADDR macros. If one of them is
not defined, then a default identity function is provided.
To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").
Fixes#1701.
