Export zinc::std for now

As demonstrated in #146 this is sometimes useful even outside of the
`zinc` crate.
k20: Add basic watchdog support.
app_blink_k20: Init watchdog on start.
k20/watchdog: Use helper instead of inline asm.
Merge pull request #150 from bgamari/master

Export zinc::std for now

Reviewed-by: farcaller
ioreg/getter: Clean up redundant lifetimes
ioreg/setter: Clean up redundant lifetimes
ioreg/accessors: Clean up redundant lifetimes
ioreg/union: Clean up redundant lifetimes
ioreg/register: Clean up redundant lifetimes
ioreg: Clean up redundant lifetimes
Merge pull request #153 from bgamari/ioreg-cleanup

ioreg: Clean up redundant lifetimes
Merge pull request #151 from dpc/master

Implement basics of #144 (watchdog)
ioreg: Allow outer doc comments on fields
ioreg::union: Use a reasonable span
ioreg::register: Fix span associated with struct
ioreg::getter: Associate correct span with Getter type
ioreg::setter: Associate correct span with Setter type
ioreg::parser: Give correct span to field types
ioreg::parser: Give correct span to top level group