epee: make {read,write}_varint public, create write_{bytes,container}

[hinto-janai]

• epee_encoding::varint::{read,write}_varint() is made public
• The duplicated inner implementations of some EpeeValue objects are extracted into epee_encoding::write_{bytes,iterator} and made public and generic

For example, a custom EpeeValue implementor can now do:

impl EpeeValue for CustomStr {
    /* ... */

    fn write<B: BufMut>(self, w: &mut B) -> Result<()> {
        epee_encoding::write_bytes(self, w)
    }
}