Remove Self: Type bounds in Encode / Decode implementations

[jplatte]

Notes on the first commit:

• This changes a few panic!s behaviors. I'm assuming not using a format string there was an oversight, rather than those panics really being meant to be catch-able as std::io::Errors.
• It looks like enter_runtime for tokio was actually wrong before, due to the runtime guard being dropped before calling the function argument.

About the second ("main") commit: I've only tested that the change of the slice implementation doesn't result in compilation errors, nothing more.

[jplatte]

This allows one to work around #298 like this:

#[derive(sqlx::Encode)]
struct Foos<'a>(&'a [Foo]);

impl sqlx::Type<sqlx::Postgres> for Foos<'_> {
    fn type_info() -> PgTypeInfo {
        PgTypeInfo::with_name("_foo")
    }
}

query_as!(
    Whatever,
    "<QUERY with $1 of type foo[]>",
    Foos(&foo_vec) as _,
)

[mehcode]

Very cool. Nice stuff @jplatte.

[pintert3]

Thank you very much for this. At least it eases some hardships

[pintert3]

Does this work for Decode implementations? I'm getting an error :

thread 'main' panicked at 'internal error: entered unreachable code: (bug) use of unresolved type declaration [kind]', /home/pintert3/.cargo/registry/src/github.com-1ecc6299db9ec823/sqlx-core-0.5.5/src/postgres/type_info.rs:744:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[jplatte]

Sounds like you probably used the wrong type name. Is the postgres type a custom one and did you prefix it with an underscore in PgTypeInfo::with_name()?

[pintert3]

Ya, I did. Well to give more context, I made an adhoc type from this query:

        select people.*, (select array(select c from cars c where c.id = $1)) "cars_owned:_" from people where people.id = $1

$1 was taken from a variable id:i32.
So I created a an output type only for getting the result of the query