Correctly constrain ValueRef Deserialize impl to allow using #[derive(Deserialize)] on types that contain it

[nabijaczleweli]

This type

#[derive(Deserialize, Debug, Clone)]
pub struct PartialMsgPackLine<'raw>(pub UuidStr, #[serde(borrow = "'raw")] pub MsgPackExt<'raw>, #[serde(borrow = "'raw")] pub rmpv::ValueRef<'raw>);

fails to typecheck with

error: lifetime may not live long enough
   --> src/collector/retina.rs:367:98
    |
366 | #[derive(Deserialize, Debug, Clone)]
    |          ----------- lifetime `'de` defined here
367 | pub struct PartialMsgPackLine<'raw>(pub UuidStr, #[serde(borrow = "'raw")] pub MsgPackExt<'raw>, #[serde(borrow = "'raw")] pub rmpv::ValueRef<'raw>);
    |                               ---- lifetime `'raw` defined here                                  ^ requires that `'raw` must outlive `'de`
    |
    = help: consider adding the following bound: `'raw: 'de`

This is because the Deserialize impl is overconstrained: it only allows deserialising into an identical lifetime instead of identical-or-stronger. For example, it only allows deserialising &'static [u8] into ValueRef<'static>, but not into ValueRef<'_>. Relax this appropriately. The code above typechecks and works with this.