lang: ast, interfaces, interpret: Table should be a well-known type

We use this in enough places, that it's nice to have it as a well-known
alias.

lang/interfaces/ast.go

@@ -94,7 +94,7 @@ type Stmt interface {
 	// Output returns the output that this "program" produces. This output
 	// is what is used to build the output graph. It requires the input
 	// table of values that are used to populate each function.
-	Output(map[Func]types.Value) (*Output, error)
+	Output(Table) (*Output, error)
 }
 
 // Expr represents an expression in the language. Expr implementations must have

lang/interfaces/func.go

@@ -44,6 +44,9 @@ import (
 // FuncSig is the simple signature that is used throughout our implementations.
 type FuncSig = func(context.Context, []types.Value) (types.Value, error)
 
+// Table is the type of language table fields.
+type Table = map[Func]types.Value
+
 // GraphSig is the simple signature that is used throughout our implementations.
 // TODO: Rename this?
 type GraphSig = func(Txn, []Func) (Func, error)