applet.memory.prom: add health histogram command. With the accompanying tool.

applet.memory.prom: print values of unstable words. This can help e.g. with distinguishing signal integrity issues from bit rot issues.

applet.memory.prom: add basic write support.

software: require Python 3.7 via setuptools.

applet.memory.prom: improve help, mention in README.

applet.memory.prom: make address and length optional. The entire PROM is read or verified if these are not specified.

applet.memory.prom: s/decayed/unstable/

applet.memory.prom: add health sweep command.

applet.memory.prom: --file is required in the verify command.

applet.memory.prom: add health {check,scan} commands.