Update derive_more when it is ready

[azriel91]

Spawned out of #108

In order to reduce the number of compilations for syn of different versions, it would be useful to upgrade derive_more 0.15, as it depends on syn 0.15. However derive_more 0.99 which uses syn 1.0 is not usable right now because:

• For derive_more::From, multiple variants with the same fields will have conflicting implementations. In 0.15 it simply didn't generate the impl From<_>. Don't derive From by default for variants without fields JelteF/derive_more#106
• Getting around that issue (Derive From for Enum has strange defaults or might be a regression JelteF/derive_more#105) requires a lot of attributes, which is undesirable:

#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, From)]
pub enum Decoration {
    #[from(ignore)]
    RelaxedPrecision,
    #[from(ignore)]
    SpecId(u32),
    #[from(ignore)]
    Block,
    #[from(ignore)]
    BufferBlock,
    #[from(ignore)]
    RowMajor,
    #[from(ignore)]
    ColMajor,
    #[from(ignore)]
    ArrayStride(u32),
    // .. etc
}

[azriel91]

This isn't really actionable, more of a record so we know why we're on an older version of syn / derive_more.