Fix error types

[Disasm]

core::convert::Infallible was stabilized in 1.34.0. This PR replaces Void and () error types with Infallible.

Note that this is a breaking change.

[rust-highfive]

r? @therealprof

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[thejpster]

What's our stated MSRV?

[ryankurte]

@thejpster huh, looks like we haven't documented one in this repo yet?

a few thoughts without particular direction:

• i wonder what the migration path for the void package is, it looks like there was a suggestion they move to '!' some time ago
• it wouldn't let void be removed completely, but, might be possible instead to add a second shim with the Infallible type to digital::v2_convert so older implementations would continue to work (until void or aliases change anyway)
• Infallible looks to be a temporary measure prior to stabilisation of !, does it make sense to switch now or later?

[Disasm]

@ryankurte As for me, Infallible is a nice replacement for Void: it's pretty the same and doesn't require an additional crate.
Anyway, my major concern was () error type in v2_compat: it doesn't provide a hint to the compiler that error condition never happens.

[thejpster]

Given our policy:

The MSRV shall always be strictly smaller than the latest available minor stable release. For example, if the latest stable Rust version was 1.32.1 a new minor release of the cortex-m crate can't bump the MSRV higher than 1.31.

And that this only requires 1.34, I'm happy to merge this. Infallible will alias ! when it stabilises in 1.40, so we can take a PR to move to ! after 1.41 is out.

[thejpster]

@rust-embedded/hal any objections to merging?

[therealprof]

Let's do this.

[therealprof]

bors r+

[bors]

Build failed

• continuous-integration/travis-ci/push

[therealprof]

Ah, the joy of requiring a funtioning nightly build.

[Disasm]

Cargo.lock is quite outdated, retrying
bors r=therealprof

[bors]

Build succeeded

• continuous-integration/travis-ci/push

[ryankurte]

PR void to assist with interoperability while we're migrating things: reem/rust-void#20