incr: making embedded-hal optional

[explocion]

A simple way to fix #821

[Dirbaio]

you can mix crates using 1.0.0-rc1 and 1.0.0-alpha.11 perfectly fine, they don't conflict at all. Rust considers them separate traits, so you can't use them together, but it still compiles.

Making e-h optional doesn't help, because it also means you still can't use these crates together.

[explocion]

error: failed to select a version for embedded-hal.
... required by package stm32f4xx-hal v0.17.1
... which satisfies dependency stm32f4xx-hal = "^0.17.1"
versions that meet the requirements =1.0.0-alpha.11 are: 1.0.0-alpha.11

all possible versions conflict with previously selected packages.

previously selected package embedded-hal v1.0.0-rc.1
... which satisfies dependency embedded-hal = "^1.0.0-rc.1" of package rtic-monotonics v1.2.0
... which satisfies dependency rtic-monotonics = "^1.2.0"

failed to select a version for embedded-hal which could resolve this conflict

I guess the cargo cannot resolve this by default.