Replace most of .inspect() (and datasette inspect) with table counting

[simonw]

This is the last part of #419 - with the move to supporting mutable databases by default, the inspect-data mechanism currently in use no-longer makes much sense.

The one optimization I think it's worth keeping for databases opened in immutable mode is the cached table counts. I think datasette inspect should cut down to only counting the rows in the tables - the other things done by inspect (figuring out columns, foreign key relationships, FTS etc) should all be fast enough that they can be reliably performed at runtime even against large databases.

If performing them at run-time has performance issues, I would rather cache those results internally within Datasette after they are first calculated than continue to support them in the datasette inspect command - to keep things simpler.

[simonw]

test_inspect.py currently just contains two tests that exercise a small portion of what .inspect() does - I'm going to repurpose that module and have it only test the datasette inspect CLI command instead.

Here's the current contents of that file: https://github.com/simonw/datasette/blob/ce09e5d2d392634eced44c3c8d603d7c628e2822/tests/test_inspect.py

[simonw]

So I think datasette inspect fixtures.db other.db should output something like this:

{
  "fixtures": {
    "hash": "894870db97229e9e18b40921dc32b581da813465d672445e96e040ab2adbd229",
    "file": "fixtures.db",
    "size": 225280,
    "tables": {
      "facetable": {
        "count": 34,
      }
   }
}

It currently writes it out to a file called inspect-data.json. Should I keep that as the default behaviour or switch it to outputting to stdout instead?

Here's the current datasette inspect --help:

Usage: datasette inspect [OPTIONS] [FILES]...

Options:
  --inspect-file TEXT
  --load-extension PATH  Path to a SQLite extension to load
  --help                 Show this message and exit.```

[simonw]

I'm going to change it to output to stdout unless you pass it the --inspect-file argument.

[simonw]

I now need to update datasette serve ... --inspect-data=X to understand and correctly handle the new format.