Fix Decimal serdes

[qhelix7]

Use string for serializing and deserializing Decimal type data in order to avoid data corruption caused by casting to float.
See Issue 188.

[seandstewart]

Not sure why the test workflow didn't run, I'll have to check the actions configuration, but the change is straightforward enough.

I ran the test suite locally and the doctest for isdecimaltype failed - you'll need to add this new check to __all__.

Otherwise this looks great!

[qhelix7]

Not sure why the test workflow didn't run, I'll have to check the actions configuration, but the change is straightforward enough.

I ran the test suite locally and the doctest for isdecimaltype failed - you'll need to add this new check to __all__.

Otherwise this looks great!

Cool. isdecimaltype is now included in __all__.

[seandstewart]

Thanks for your contribution @qhelix7! I'll make sure to get this out in the next release (probably later today or tomorrow).