Float16 support

[aldanor]

Closes #236 (?)

[aldanor]

• We now run roundtrip tests for both f16 and complex
• Refactored DynValue in this PR as well, packing floats into one variant, now that there's (up to) 3 of them
• "complex" and "f16" are now top-level features in the main crate
• Doc builds now have f16/complex enabled
• Enabled f16/complex in at least one build, the static one (features in the CI are a complete mess...)

[aldanor]

@mulimoen In the failures, your test_issue_223 test has failed apparently 🤔

(btw I moved the tests under hdf5/ now, so previously things might have been running slightly incorrectly...)

[mulimoen]

With the small nits and #247 merged this looks great!

[aldanor]

Enabled f16/complex for docs.rs builds for both hdf5 and hdf5-types - thanks, good call.

As you said above, exhaustive attr might not be the best idea since it would force ourselves to panic when matching which is a bit weird.

If it builds green, I think it should be good to go :)

[mulimoen]

Looks good to go if green!