Add count_min_sketch aggregate and approx_count accessor

[rtwalker]

Add count_min_sketch aggregate and approx_count accessor.

The initial implementations work only on text columns

Resolves #373

[WireBaron]

This is the antipattern that is my biggest grievance with how we use the crates subdirectory. Converting the Postgres CountMinSketch object back into the crate object so that we can call a function on it is really not efficient. We can leave this in place for now, but we'll need to implement the estimate functionality directly on top of the Postgres structure.

[epgts]

Having implemented this two or three times so far myself, I couldn't agree more that it's a mess! But I think the answer is to break extension up into more crates, so that we can build business logic directly against pg_type structs without all this destruction and reconstruction and so that we can avoid giving rustc an ever-growing pile of files to handle when I make a one-line change to only one of those files. What stops us now is so much of what goes into a pg_type is locked inside the extension crate.

We've nibbled at the edges of this discussion a few times... how about I spend a day to prototype something like this when I get back from vacation?

Thanks!

[rtwalker]

I'd be interested to see that!

[WireBaron]

Looks good, just one last suggestion.

[rtwalker]

bors r+

[bors]

Build succeeded:

• Clippy Test
• Test Postgres (12)
• Test Postgres (13)
• Test Postgres (14)