Port over existing peripheral drivers from esp32-hal

[jessebraham]

There are a number of peripheral drivers already implemented in the esp32-hal repository. Since this crate has been published, we aren't really able to release a new version until we've roughly reached feature-parity with the previous implementation. Because of this, we should begin working on porting this drivers.

These can either just be dumped in the esp32-hal package for now, making the required changes just to get things building, or can be re-written to be compatible across all supported chips. While the latter method is preferable, the prior is likely the more realistic path forward.

The existing drivers are:

• analog/adc
• analog/dac
• clock_control see We need a way to configure clock rates #44
• delay
• dport
• dprint
• efuse
• external_ram
• gpio
• i2c
• interrupt
• ledc (in progress)
• mem
• serial
• spi
• timer (partially complete)

[bjoernQ]

I think dprint shouldn't get ported since we have esp-println for that.

For clock control we also already have issue #44

I'm not sure if this really should be a hard requirement for a potential release. Someone using the deprecated HAL can just pin the version and even more important even if we have all of those the new HAL isn't a drop-in replacement since many things are just different (for good reasons).

But certainly, it would be a good thing to cover all of those

[bjoernQ]

We have support for timer - what we should do however is handling it similar to SYSTIMER and split each timer to make use of the three alarms. But I'd say that is a "nice-to-have" enhancement

I'm not sure about that mem feature - while it's easy to copy over we should actually benchmark that it is really an improvement (I doubt it is - see https://github.com/rust-lang/compiler-builtins/blob/master/src/mem/impls.rs )

[jessebraham]

I'm going to close this. Other than LEDC (which is in progress and hopefully will be merged in the coming weeks) there's not really anything left worth worrying about right now, I don't think.