Use absolute paths in derive macro

[novafacing]

Uses absolute paths in derive-macro generated code to permit not using top-level use statements.

This permits things like:

#[derive(nanoserde::SerJson, nanoserde::DeJson)]
pub struct Foo {
  val: String,
}

Without having a use nanoserde::{SerJson, DeJson} at global scope.

This would solve #45 for my use case.

[knickish]

Thanks for working on this. I'm concerned that hard coding these paths will prevent the derive macros from being used if the crate is renamed in Cargo.toml, which is a use case that has been supported so far. I would lean towards accepting this if an override similar to #[serde(crate = "renamed_serde")] was also present, as there's still time to add that before v0.2. Any opinions on that @not-fl3?

[novafacing]

I went ahead and added the ability to rename the crate like so (better example in the added tests).

use nanoserde as other;
#[derive(other::DeJson)]
#[nserde(crate = "other")]
struct Foo;

[knickish]

I'd like to wait and see if @not-fl3 has any opinion on it before merging, but that looks great to me, appreciate it.

[not-fl3]

Looks great to me too!