On SOLID, the conversion functions are also used to implement helpers for
timeout conversion, so these stay in the PAL. The `Instant` (µITRON) and
`SystemTime` (SOLID-specific) implementations are merged into one. While it was
nice to have the µITRON parts in a separate module, there really isn't a need
for this currently, as there is no other µITRON target. Let's not worry about
this until such a target gets added...
Note that I've extracted the `get_tim` call from `Instant` into a wrapper
function in the PAL to avoid the need to make the inner `Instant` field public
for use in the PAL.
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