remove form dependency

[tones111]

This is another stepping stone toward #157. I did not see any difference in build time on my machine when running cargo check --all-features.

It probably needs the same CI changes to build the example as #176, but I didn't want to duplicate them here. This change can be rebased once that one lands.

[Rahix]

Hi,

I am not sure I understand the intention of this PR. #157 will drop form anyway because it removes all things referencing it. So this PR would just generate merge conflicts for #157?

Also, removing form without a replacement does not make sense to me either - as long as #157 is not yet merged, we want to have form to cleanup up the file structure, no?

[tones111]

By landing this change upstream we reduce the complexity and diff that needs to be analyzed when evaluating #157 which has already shown to be too complicated. I think it should focus on the Makefile -> build.rs migration as we can iron out these pieces separately.

I'm not sure I understand the motivation to run form against the generated mcu code. Several years ago the compiler and rustfmt would choke (take several minutes) on the large modules out of svd2rust, but today the compile time is the same. So I don't know what benefits we get from splitting the file into many modules. I'm assuming the generated documentation is the same either way.

Update:
After generating docs before and after this change (1.9GB for --all-features) the only differences I'm seeing are the source links that now point into the per-mcu rs file instead of the broken out file structure.

Example:
before: href="../../../../src/avr_device/devices/at90usb1286/ac/acsr.rs.html#165-167"
after: href="../../../../src/avr_device/devices/at90usb1286.rs.html#350-352"

[tones111]

#157 is getting close, so defer to that solution