Rewrite SWJ config when modifying MAPR (fixes #105)

[TheZoq2]

This stack exchange post explains the issue pretty well https://electronics.stackexchange.com/questions/260757/timer1-remap-cause-debug-crash-on-stm32f103

The SWJ_CFG bits are write only, meaning that reads are undefined. On the blue pill, reads seem to work fine anyway, but on the RGT6, they return some other value which messes with the debug port.

My fix for it is to make the mapr function private, and only allow modifications via the modify_mapr function which remembers the state of the debug port and writes the config accordingly.

Since mapr was public, this is a breaking change. I see no reason to have it public, in fact, users can break the abstractions by modifying mapr so this new modify_mapr function is marked pub(crate)

[geomatsi]

LGTM, thanks !

You probably mean 'SWJ_CFG bits' (not pins) in commit message. FWIW, suggested approach is extensible in terms of support of other debug port mappings. In other words, when more SWJ_CFG combinations are needed, jtag_enabled: bool can be replaced by appropriate enum.

Regards,
Sergey

[TheZoq2]

FWIW, suggested approach is extensible in terms of support of other debug port mappings

That's true, but since this is internal stuff, I'd say we can make that change if we decide to support other modes

[therealprof]

LGTM