Unify type signature declaration syntax

[lukaszcz]

• Closes Unify syntax for named arguments in traits, instances, and functions #2990
• Allows type signatures for constructors, record fields and axioms to have the same form as in function definitions, e.g.,

field {A} {{M A}} (arg1 arg2 : A) : List A

• For constructors, this introduces an ambiguity between record and GADT syntax with names. For example,

ctr {
  A : Type
}

can be confused by the parser with initial part of GADT type signature:

ctr {A : Type} : A -> C A

For now, this is resolved by preferring the record syntax. Hence, it's currently not possible to use type signatures with implicit arguments for constructors. Ultimately, the @ in record syntax should be made mandatory, which would resolve the ambiguity:

ctr@{
  A : Type
}