hal: Switch from custom time traits to `embedded_time`

[twitchyliquid64]

WIP

Anyone got a thumbv7-based board they can test the timers/pwm on to make sure the new divider calcuations are correct?

[jacobrosenthal]

Took a look. PWM (tc4 tcc0) pygamer looked consistent. But timer tc3 panics

18:51:35.272 panicked at 'internal error: entered unreachable code',
/home/jacob/Downloads/atsamd/hal/src/common/thumbv7em/timer.rs:107:22

I havent dug in further yet

[jacobrosenthal]

embedded-time got some nice cleanups after rtic started implementing it . Might be worth another look

[TDHolmes]

there's also now fugit which I like (it's a lot simpler), but it doesn't have the Hz()/kHz()/etc. helpers AFAIK.

[AfoHT]

Latest release fugit v0.3.5 added support for Hz, kHz etc :D

[jbeaurivage]

Closed via #672.