Flatten HAL and move common content to crate root

[glaeqen]

WiP: I published this PR in parallel to #424 for review purposes. Preferably #424 is going to be merged in first; thus, samd{1,2}1 modules will be absent and HAL structure will become much cleaner.
#424 is already in.

• Incorporate common/mod.rs into lib.rs
• Create a shim implementation of common module inside of lib.rs for backward compatibility
• Removed common/mod.rs
• Moved common/* to crate root

Long term goals note:

• Get rid of the shim as soon as API breakage is acceptable (BSCs + examples are reworked)
• Make other implementation-leaking modules (thumbv6m, thumbv7em) private

[jessebraham]

This looks good to me, thanks! I'd just like to have another set of eyes on it before merging due to the number of changes made, just to be safe.

[bradleyharden]

@jessebraham, should we move forward with this?