Mark various internal functions as unsafe

[rytheo]

Found various functions that should be unsafe since they take raw pointer arguments and must assume they point to valid data.

Made string_from_fixed_bytes panic on invalid UTF-8 instead of marking it unsafe, but marking it unsafe is a valid alternative if the unwrap call significantly reduces performance.

[mulimoen]

Looks good, this will allow storing C functions directly in the type without wrapping it up in a "safe" function.

Marking the functions in hdf5/src/util.rs as unsafe is a breaking change. Could this be added to the changelog?

[rytheo]

@mulimoen changing hdf5/src/util.rs shouldn't be a breaking change; looking at hdf5/src/lib.rs, it seems like none of the functions in util are accessible outside the crate. Am I missing something?

[mulimoen]

@rytheo You are right, after closer examination it is not exported publicly. Note to self: Should mark these functions/modules as pub(crate)