graph, store: bump num-bigint and bigdecimal

[vrmiguel]

This PR bumps

• num-bigint from ^0.2.6 to ^0.4.7
• bigdecimalfrom 0.1.0 to 0.3.0

This change has pros and cons, however.

Pros

• These types have had many optimizations since the versions we're currently using were released:
  • Reduce the computational complexity of formatting rust-num/num-bigint#216
  • Optimizations for multiplication rust-num/num-bigint#199
  • Shrink overallocated capacity if a number gets a lot smaller. rust-num/num-bigint#171
  • Implement shift with broad RHS types rust-num/num-bigint#142
  • Use the _addcarry and _subborrow intrinsics when available rust-num/num-bigint#141
  • Finish i128 support rust-num/num-bigint#62
• We can now use upstream's implementation for BigDecimal::normalized.
• bigdecimal no longer implements From<f64> for BigDecimal, which is something that could panic before.
  This means that the graph-node-defined scalar::BigDecimal struct now implements TryFrom<f64> instead of From<f64>

Cons

• The latest Diesel release still uses num-bigint 0.2.6 and bigdecimal 0.1.0.
  This means that we're no longer able to use the Diesel-defined ToSql<diesel::sql_types::Numeric, _> and FromSql<diesel::sql_types::Numeric, _> for BigDecimal implementations for version 0.3.0 of BigDecimal until Diesel upgrades their dependencies.
  In order to maintain support for these traits, I implemented the BigDecimalRetrocompatibility struct, which has functions for converting to and from versions 0.1 and 0.3 of BigDecimal.
  API limitations make this operation make heap allocations, although I believe those allocations should be small most of the time.

@tilacog @lutter @otaviopace @leoyvens
If you guys believe that this PR is the way to go, I'll add tests for BigDecimalRetrocompatibility and further docs on things I changed

[leoyvens]

Diesel 2.0 will upgrade these dependencies diesel-rs/diesel#2883 so we can choose to wait for that so we don't need the compatibility tricks.

[evaporei]

@vrmiguel @leoyvens will this be continued? Otherwise I think we should close it.

[leoyvens]

Closing, we should upgrade this along with Diesel 2.0 which will be released soon.