Add CapturePin trait

[YuhanLiin]

The CapturePin trait is the input capture analogue to the PwmPin trait, representing a single capture channel. Unlike PwmPin, there's no enable or disable methods, because some chips, such as the MSP430FR2355, may have trouble implementing those two methods on capture abstractions. The proposed trait has been implemented in msp430fr2x5x-hal.

[rust-highfive]

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[YuhanLiin]

The proposal for the trait is here, which I added after reading that I need to complete a proposal before opening a PR to add a new trait.

[YuhanLiin]

Any updates on this?

[eldruin]

I would say let's reevaluate this after the 1.0 release.