Add manifest to levelup and abstract-leveldown implementations

[vweevers]

Background and discussion: #42.

Add manifest to:

• abstract-leveldown
  • PR or commit: Add manifest abstract-leveldown#351
  • Release: 6.2.0
• encoding-down
  • PR or commit: Add manifest encoding-down#93
  • Release: 6.3.0
• leveldown
  • PR or commit: Add manifest leveldown#685
  • Release: 5.4.0
• deferred-leveldown
  • Related: implement manifest functionality deferred-leveldown#35
  • PR or commit: Add manifest deferred-leveldown#79
  • Release: 5.3.0
• levelup
  • PR or commit: Add manifest levelup#678
  • Release: 4.3.2
• memdown
  • PR or commit: Add manifest memdown#198
  • Release: 5.1.0
• level-packager
  • PR or commit: Level/packager@cd22e66
  • Release: 5.1.0
• level-js
  • PR or commit: Add manifest level-js#183
  • Release: 5.0.0
• level
  • PR or commit: Level/level@97cb31c, Document manifest, iterator() and clear() level#162
  • Release: 6.0.0
• level-mem: 6.0.0
• rocksdb
  • PR or commit: Level/rocksdb@5eee577
  • Release: 5.0.0
• level-rocksdb
• subleveldown
  • PR or commit: Level/subleveldown@f7ee1e3
  • Release: 6.0.0
• multileveldown (and by extension, level-party)
  • PR or commit: Level/multileveldown@b331481
  • Release: 5.0.0
• leveldown-hyper and level-hyper (slated to be deprecated)