[tracking] ZeroVec improvements

[Manishearth]

We have a whole bunch of ZeroVec improvements we want to do, this issue exists to track them.

⭐'d entries are planned for Q4 2021, the rest are "nice to have" and not currently planned with any specific target

• ⭐ Support for infinitely nesting VarZeroVecs #1065
  • ⭐ Restructure AsVarULE or get rid of it #1078 (needed by @zbraniecki for PluralRuleStringsV1 should be parsed at data load time #615)
    • ⭐ Converting to VarULE #1179 (not needed super strongly just yet)
      • ⭐ Basic impl of EncodeAsVarULE #1173 (needed by @zbraniecki for DTF)
      • ⭐ Add more methods to EncodeAsVarULE #1199
    • ⭐ Add borrowed-only versions of data structs in ZeroVec #676 (needed by @sffc for Migrate BlobProvider to ZeroMap #1056) sub-issues:
      • ⭐ VarZeroVecBorrowed Make VarZeroVecBorrowed/VarZeroVecOwned public #1223
        • Improve docs for owned/borrowed variants of VarZeroVec #1228
      • ZeroVecBorrowed Deemed unnecessary for now
      • ⭐ ZeroMapBorrowed Add borrowed-only version of ZeroMap #1238
        • Cleanup: Make ZeroMapKV simple to implement #1247
    • Proc macros for AsULE and AsVarULE #1079
      • Depends on Simplify ZeroVec/VarZeroVec error handling, consolidate ULEError type #1389
      • Use zerovec proc macros where possible, get rid of num_enum dep #1642
      • Consider integrating zerovec into rkyv's serialization benchmark #1258
  • ⭐ Improve mutation operations on VarZeroVecOwned #1080
• Fallible operations in zerovec #1081
• ⭐ Allow equality to be byte equality on VarZeroVec #1083 (PR at Use/require byte equality in VarZeroVec #1103)
• ⭐ Move zerovec to no_std #1094
• Fail with length error in PlainOldULE, CharULE, etc. #1144
• ⭐ Consider requiring all ULEs to be Copy #1184
  • Rename as_unaligned to to_unaligned #1260
• Should we have FromVarULE? #1180
• Investigate the possibility of u16 lengths in VarZeroVec #1261
• Rename [Var]ZeroVecULE to [Var]ZeroSlice, use Deref #1367
• Refactor VZV code to center on (and Deref to) VarZeroSlice; make VZVBorrowed private #1418
• ZeroVec documentation/polish #1475
  • contains: Figure out public module structure for zerovec #1190
• Consider FlexZeroVec #1443

cc @sffc @zbraniecki

[Manishearth]

The items planned for the 0.5 milestone were completed, removing the tracking issue from the milestone. Everything else is mostly a "nice to have"

[Manishearth]

There's still work to be done here but the initial set of planned things is done including all stretch items and I don't think we need this megaissue anymore.