Support split signatures

[fizruk]

Currently, mkSignature generates a single signature, but it might be useful to that into multiple smaller signatures at user's request. For instance,

-- generate signature for the entire lambda-pi language
mkSignature ''Raw.Term' ''Raw.VarIdent ''Raw.ScopedTerm' ''Raw.Pattern'

-- partial signature covering only functions and the universe type
mkPartialSignature "Functions"
  ''Raw.Term' ''Raw.VarIdent ''Raw.ScopedTerm' ''Raw.Pattern'
  ['Raw.Universe, 'Raw.Pi, 'Raw.Lam, 'Raw.App]
  
-- partial signature covering only pairs
mkPartialSignature "Pairs"
  ''Raw.Term' ''Raw.VarIdent ''Raw.ScopedTerm' ''Raw.Pattern'
  ['Raw.Product, 'Raw.Pair, 'Raw.First, 'Raw.Second]

Partial signatures are naturally embedded into full signatures, so it is possible to easily go between the AST representations.