Meta: add builtins

[Anton-Latukha]

Which are left:
Important:

• Implement builtins.zipAttrsWith #1055
• Implement builtins.fetchGit #1034
• Implement builtins.filterSource #233
  Parts of implementation of it should be in Support uploading filtered paths to the store #755
• Implement builtins.storePath #240
• Implement builtins.builtins #273
• Implement builtins.fetchTarball through Haskell #162
• Implement builtins.path #128
  There is implementation of it in Support uploading filtered paths to the store #755
  Ported in Builtins: add path & unsafeDiscardOutputDependency #1032
• Implement builtins.groubBy Builtins: add groupBy #1033
• Implement builtins.unsafeDiscardOutputDependency
  There is implementation of it in Support uploading filtered paths to the store #755
  Ported in Builtins: add path & unsafeDiscardOutputDependency #1032
• Implement builtins.ceil (#1031) Builtins: add floor & ceil #1031
• Implement builtins.floor (#1031) Builtins: add floor & ceil #1031

Of smaller importance:

• builtins.fetchMercurial
• builtins.fromTOML

This is currently it.

[Anton-Latukha]

I already futurePorted unsafeDiscardOutputDependency, and now trying to port path.

[Anton-Latukha]

Also ported path.

[soulomoon]

I am porting groubBy now.

---

groubBy ported