feat: expose various low level memory management menthods

[adriangb]

I'm diagnosing memory usage in an application, I think these would be useful to have.

[abonander]

The very first line of the linked documentation says:

These interfaces impose limits on the amount of heap memory that will be by all database connections within a single process.

So saying it sets it for the current thread, or having it as a method on the connection. is very misleading.

It should be a free function, or perhaps an associated method of the Sqlite type. Probably a free function.

Passing a negative number just gets the heap limit and doesn't set it, which also needs to be documented.

A more Rust-y API would be something like limit: Option<NonZeroU64>, returning an error if it's greater than i64::MAX (or using a wrapper type with fallible constructors), and providing a separate getter function, but I don't know if we need to go that far. I'd kind of like to sleep on it, though.

[abonander]

This links to the wrong page. It should be: https://www.sqlite.org/c3ref/db_release_memory.html

[abonander]

@adriangb do you have time to rebase this and fix the CI failures? There are lint errors that need to be addressed:

error: casting `u64` to `i64` may wrap around the value
   --> sqlx-sqlite/src/lib.rs:155:13
    |
155 |             value as i64
    |             ^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap
    = note: requested on the command line with `-D clippy::cast-possible-wrap`

error: casting `i64` to `u64` may lose the sign of the value
   --> sqlx-sqlite/src/lib.rs:163:40
    |
163 |         Some(std::num::NonZeroU64::new(previous as u64).unwrap())
    |                                        ^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_sign_loss
    = note: requested on the command line with `-D clippy::cast-sign-loss`

error: casting `i64` to `u64` may lose the sign of the value
   --> sqlx-sqlite/src/lib.rs:180:40
    |
180 |         Some(std::num::NonZeroU64::new(current as u64).unwrap())
    |                                        ^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_sign_loss

[abonander]

Closing to clear out the PR queue. Feel free to open a new PR after rebasing and fixing CI failures.