Use primitive formatting for primitive references

[Sh3Rm4n]

Closes #163

A single recursive call in as_native_type is sufficient
to unpack the type from the reference.
Formatting functions (fmt.i32() etc.) are already taking
a reference, so this does not need to be changed.

Tests are adjusted, so that they pass.
This introduced change is not explicitly tested though.

#[derive(Format)]
struct S2<'a> {
    x: &'a i32,
    y: &'a u8,
}

now expands into

fn format(&self, f: &mut defmt::Formatter) {
    if f.needs_tag() {
        f.istr (&defmt::export::istr({ 
            #[link_section = ".defmt.{\"package\":\"use-test\",\"tag\":\"defmt_fmt\",\"data\":\"S2 {{ x: {:i32}, y: {:u8} }}\",\"disambiguator\":\"10404396863279507298\"}"]
            #[export_name = "{\"package\":\"use-test\",\"tag\":\"defmt_fmt\",\"data\":\"S2 {{ x: {:i32}, y: {:u8} }}\",\"disambiguator\":\"10404396863279507298\"}"]
            static S: u8 = 0; 
            &S as * const u8 as usize 
        })) ;
    }
    match self {
        Self { x, y } => {
            f.i32(x);
            f.u8(y);
        }
    }
}

[japaric]

Nice! This looks good.

This introduced change is not explicitly tested though.

(the unit test fixed in this PR is testing the derive macro change, at least it shows that the encoding is now smaller)

The CI/nightly build is broken for reasons unrelated to this this PR. We should figure out some workaround for that ...

[japaric]

bors r+

[bors]

Build succeeded:

• ci