do not merge - wip

[iamjpotts]

Explores introducing an IntoEncode trait with two methods - one which returns an Encode with the same lifetime, and another which returns an Encode with a 'static lifetime, for future use with named parameter support.

Relevant commit is the last one with commit message wip - IntoEncode

All other commits are a part of open and draft prs preceding this one.

Has unresolved compile errors related to borrowing/lifetime problems for some query bind parameters.

The error which shows up with Option usage (here with Some borrow of string):

error[E0597]: `string` does not live long enough
   --> tests/sqlite/macros.rs:189:14
    |
182 |       let string = "Hello, world!".to_string();
    |           ------ binding `string` declared here
...
185 |       let result = sqlx::query!(
    |  __________________-
186 | |         "SELECT 'Hello, world!' as string where 'Hello, world!' in (?, ?, ?, ?, ?, ?, ?)",
187 | |         string, // make sure we don't actually take ownership here
188 | |         &string[..],
189 | |         Some(&string),
    | |              ^^^^^^^ borrowed value does not live long enough
...   |
193 | |         tuple.0 // make sure we're not trying to move out of a field expression
194 | |     )
    | |_____- argument requires that `string` is borrowed for `'static`
...
201 |   }
    |   - `string` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.