Fix for Issue #2549 - cannot use feature "rust_decimal" without also using "bigdecimal"

[deneut]

Fix for this issue.

When trying to use the (newly renamed) rust_decimal feature in version 7.0.0, compiling fails because there is a missing dependency on num-bigint and numerous places in the code where the previous feature name of decimal was not updated to the new name of rust_decimal.

Even if one includes the bigdecimal feature when using the current code on main, rust_decimal functionality is not restored (although it is then possible to compile) because the macros still return BigDecimal for all NUMERIC Postgres types.

This small patch fixes those issues and restores rust_decimal functionality.

[Jezza]

Bump.

I'd love for this to get merged. :D

[cemoktra]

Yes this blocks migrating to 0.7

[cemoktra]

@abonander will there be a 0.7.1 soon?