Fix #38 - support `rust_decimal`.

[finnbear]

Fixes #38

[finnbear]

Remaining questions:

• Is the Flags encoding maximally efficient?
  • justification of why it is: putting the sign bit at the LSB could help reduce the overall range of u8 values, allowing packing to work
• Do all numbers round-trip properly?
  • [x] need to check non-normalized decimal (with trailing zeroes)
  • [x] strongly suspect -0 would be converted to 0 upon decoding (I want to fix this)

[SharksT]

Any update on this ? I would love to use this crate but I need rust decimal :)

[finnbear]

Well, now there are two people who want it ;)

[l-7-l]

@finnbear Maybe there are many, many people who want this. Perhaps precisely because their business now relies on rust_decimal, they currently cannot introduce bitcode.

Don't let it gather dust—merge it and let it shine! And also for uuid.

Thank you for this masterpiece

[caibear]

Before merging, I think we should

• Avoid any unsafe code in the rust_decimal.rs file. It's not maintainable for us to add new unsafe code for each library that gets added.
• Consider the Pros/Cons of this custom implementation vs a much simpler https://docs.rs/rust_decimal/latest/rust_decimal/struct.Decimal.html#method.serialize
• These 2 ensure that the implementation doesn't use bitcode internals and could exist outside bitcode if ConvertFrom and impl_convert were exposed.

[caibear]

Also I noticed that rkyv and borsh serialization avoids validating anything about the Decimal. I want to get the opinion of the rust_decimal maintainers if this is a bug or allows faster deserialization.