Remove the SystemExt trait and rename SystemParts to SystemControl

[jessebraham]

Further work towards #1401, finally removes the SystemExt trait entirely. The SystemParts struct has been renamed to SystemControl (open to suggestions on naming here, but this seemed to fit with the other structs we've defined) which now has a constructor which takes the SYSTEM peripheral.

@MabezDev I'm still not 100% sure if we're on the same page regarding these changes or not, so would appreciate your feedback here. As usual happy to make any changes.

While I was in here I also noticed that the SoftwareInterrupt struct had no fields, so converted it to a unit struct instead.

I also moved some CHANGELOG.md entries which were mistakenly added to the wrong release in another PR.

[MabezDev]

LGTM, thanks for taking care of this!

[bjoernQ]

LGTM

[JurajSadel]

LGTM, thanks!

[Ben-PH]

It seems this PR is the source of some frustration I had today, and I believe it comes down to there being no version bump:

• I was wanting to try the pcnt module.
• I copy-pasted the pcnt example code.
• Imports could not be resolved.
• lsp "goto definition" on systems module: desired SystemControl struct nowhere to be found in source
• updated my local clone of esp-hal and ran the pcnt as instructed: all good
• looked at upstream main branch and found SystemControl
• looked it up on my local updated clone, found it where expected
• changed my toml to us esp-hal = { git = "...} and that fixed the issue

My conclusion is that without the version bump, cargo does not know that it needs to refresh/update esp-hal.