Refactor VectActive to Vector and pass it to DefaultHandler.

[reitermarkus]

Changes:

• Add a Vector::map_interrupt function to allow mapping the Vector::Interrupt variant to e.g. a HAL's interrupt type.
• Add a SCB::vect_pendingfunction for getting the pending vector, if any.

[rust-highfive]

r? @thalesfragoso

(rust-highfive has picked a reviewer for you, use r? to override)

[reitermarkus]

@thalesfragoso, ping.

[jonathanpallant]

Sorry we dropped this one. It looks OK to me, but it's a breaking change.

@rust-embedded/arm how do we feel about breaking changes in cortex-m / cortex-m-rt 0.8?

[thalesfragoso]

Any reason for returning the argument back on error?

[adamgreig]

@rust-embedded/arm how do we feel about breaking changes in cortex-m / cortex-m-rt 0.8?

I don't have a problem with breaking changes in 0.8 per se, but breaking both c-m and c-m-rt at the same time in an inter-dependent way will make upgrading even harder than it already is, so the change would have to be really well motivated and I don't really understand the motivation here.