fixed postgres serialisation, deserialisation of decimal

[serejkaaa512]

Fixes #241

[paupino]

Some minor changes/feedback. I think the major thing is perhaps changing with_scale to rescale (or something similar). I do also want to run some benchmarks on this too to ensure we don't regress since it is doing a bit more boxing.

[paupino]

I think this is looking good. There are some minor adjustments I want to make in the docs, as well as making rescale mutable, however I will do that as part of the merge.
Before I finalize the merge I just want to review the rescale_to_maximum code to make sure it's both functionally equivalent as well as performant.
Thank you for working through this with me!

[paupino]

For the record, performance seems fairly consistent between the two version with this version being slightly more performant. Future performance benchmarking should be built into the PR process wherever possible.