Use semver instead of fixed minor ver for rust-embed

[anuraaga]

Also removes explicit dependency on rust-embed-impl, which is a transitive dependency of rust-embed (I couldn't find any references to it in this crate).

I am trying to use rust-vfs with a newer version of rust-embed but the 6.3.* specifier prevents this. Presumably this was meant to be a minimum version number, not hard lock, so I tried to change to the semver syntax for that.