lang: interpolate, funcs: Add a concat function for faster interpolation

This adds a concat function which can be used directly by string
interpolation to avoid having to constantly unify the plus operator
which is much slower at this time.

The new monomorphisms changes caused type unification of a notable
example to go from ~25s to ~5m30s which was obviously not bearable. With
this fix, things are now down to ~6s.

This is an important optimization, but it's also a good reminder that
type unification of polymorphic functions needs to be improved in
general too.

lang/funcs/core/concat_func.go

@@ -0,0 +1,43 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package core
+
+import (
+	"github.com/purpleidea/mgmt/lang/funcs"
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+const (
+	// ConcatFuncName is the name this function is registered as.
+	ConcatFuncName = funcs.ConcatFuncName
+)
+
+func init() {
+	simple.Register(ConcatFuncName, &types.FuncValue{
+		T: types.NewType("func(a str, b str) str"),
+		V: Concat,
+	})
+}
+
+// Concat concatenates two strings together.
+func Concat(input []types.Value) (types.Value, error) {
+	return &types.StrValue{
+		V: input[0].Str() + input[1].Str(),
+	}, nil
+}