Add PG::Numeric#to_big_d

[jgaskins]

Exploring using decimal columns for storing currency values and representing them with BigDecimal values in Crystal code, but then noticed BigDecimal support hasn't made it into this shard yet. When PG::Numeric#to_big_r was added, I don't think BigDecimal existed in the Crystal stdlib.

I copied most of the code from to_big_r, but rather than building up a power of 10 for the second argument based on the placement of the decimal point, we can just use the placement of the decimal point itself.

A couple of the tests are failing and I'm not sure how to fix them yet. They look like trivial ones, but I spent too long trying to figure out how BigDecimal` works and I'm out of gas for the evening.

[jgaskins]

A couple of the tests are failing and I'm not sure how to fix them yet.

j/k the expected outputs were wrong

[bcardiff]

Looks great.

Don't we also need a way to encode BigDecimal as a parameter?

[will]

Thanks @jgaskins ! Could you also please add a changelog item?

[jgaskins]

Don't we also need a way to encode BigDecimal as a parameter?

For #295, absolutely. Until then, is the catch-all sufficient?

Could you also please add a changelog item?

Sure thing!

[jgaskins]

Just checking in, is there anything else this PR needs before merging?

[will]

Oh whoops sorry about letting this sit for so long! I forgot to check back in. Thanks for bumping it