Make features explicit for stm32h7[45b]3

[lzrd]

Because stmh743 and stmh753 are so similar, it didn't matter that
the h743 feature and/or feature defaults that used h743 were in use
for the h753 SoC. However, as we start to use the IP of the h753 that
does not exist on the h743, i.e. Rng, Hash, and Crypt, the differences
start to matter.

The changes are tedious which reinforces the argument for better
system configuration tools.

I also added a -e option to xtask 'cargo xtask dist [--edges|-e] ...' that
runs 'cargo tree --edges features' before running 'cargo rustc ...'
This allows more visibility into the task dependencies and what features
are enabled on each. This was helpful in tracking down where the line of
features was broken; a top-level toml may have invoked a feature, but a
mid-level toml would have a dependency on a library that would have used
that feature if it had been passed on. Instead, a default feature or no
feature would be used. Most often, the stm32h7 crate would get compiled
two different ways and then get linked into the same task. The global
for the base of the register blocks would then have two instances which
would conflict.

[lzrd]

This is tested as far as successfully running 'cargo xtask dist' on every app*.toml in hubris.