Add i128 feature

[semyon2105]

Hi!

This PR adds an optional i128 feature which enables conversions between BigDecimal and i128/u128. It also enables deserialization support for these types.

I also had to replace the serde_json test dependency with serde_yaml because of incomplete support of i128 in serde_json. serde_json recognized numeric values as Serde maps when the arbitrary_precision feature was enabled, which broke tests. serde_yaml had no such problems.

[semyon2105]

It seems like serde_yaml uses a feature which was stabilized in Rust 1.17. Can we drop support for Rust 1.16 in favor of 1.17?

[rubdos]

I also had to replace the serde_json test dependency with serde_yaml because of incomplete support of i128 in serde_json. serde_json recognized numeric values as Serde maps when the arbitrary_precision feature was enabled, which broke tests. serde_yaml had no such problems.

Do they have a tracking issue for that? Mind linking that here?

[semyon2105]

My apologies. Turns out this is not a bug but a design decision. With arbitrary_precision feature enabled, serde_json deserializes all numbers into a Number value. So there has to be an additional step: conversion from the Number value into BigDecimal. I guess I'll revert the serde_yaml dependency and add serde tests for when the arbitrary_precision feature is enabled.

We can also add an optional serde_json feature in which there will be a From<serde_json::Number> implementation for BigDecimal.

[semyon2105]

Nevermind my previous comment - serde_json purposely does not implement visit_u128 and visit_i128. Here's a related PR that was rejected: serde-rs/json#498 .

We can test deserialization of 128 ints only using the serde_yaml crate