Restructure AsVarULE or get rid of it

[Manishearth]

Part of #1082

Depends on #1065

Currently AsVarULE has an as_unaligned() that imposes the requirement that AsVarULE types contian their VarULE counterparts somehow. This is fine for the current set of VarULE types, but in general does not work for e.g. custom VarULE types like

struct Relation {
   something: u8,
   something_else: u32,
   data: VarZeroVec<String>
}

#[repr(packed)]
struct RelationULE {
   something: u8::ULE,
   something_else: u32::ULE,
   data: [u8]
}

We currently only require as_unaligned() for the PartialEq implementation, which we can get around by adding some comparison methods to AsVarULE.

At a more fundamental level, it's worth considering if AsVarULE is worth it in the first place: VarZeroVec<str> might be just as convenient. We only need AsULE for ZeroVec for convenient conversions, but VarZeroVec promises to provide usable values for a lot of its use cases. Perhaps it is better to get rid of the distinction entirely.

cc @zbraniecki @sffc

This is needed by @zbraniecki for #615

[sffc]

Related: if we change VarZeroVec<String> to VarZeroVec<str> and VarZeroVec<Vec<u8>> to VarZeroVec<[u8]>, then we also need to change ZeroMap<String, Vec<u8>> to ZeroMap<str, [u8]>. (follow-up to #1057 (comment))

[Manishearth]

Yep!