lang: types, funcs: Add simple function API

This patch adds a simple function API for writing simple, pure
functions. This should reduce the amount of boilerplate required for
most functions, and make growing a stdlib significantly easier. If you
need to build more complex, event-generating functions, or statically
polymorphic functions, then you'll still need to use the normal API for
now.

This also makes all of these pure functions available automatically
within templates. It might make sense to group these functions into
packages to make their logical organization easier, but this is a good
enough start for now.

Lastly, this added some missing pieces to our types library. You can now
use `ValueOf` to convert from a `reflect.Value` to the corresponding
`Value` in our type system, if an equivalent exists.

Unfortunately, we're severely lacking in tests for these new types
library additions, but look forward to growing some in the future!

lang/funcs/simple/int2str_func.go

@@ -0,0 +1,35 @@
+// Mgmt
+// Copyright (C) 2013-2018+ 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 simple // TODO: should this be in its own individual package?
+
+import (
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	Register("int2str", &types.FuncValue{
+		T: types.NewType("func(a int) str"),
+		V: func(input []types.Value) (types.Value, error) {
+			return &types.StrValue{
+				V: fmt.Sprintf("%d", input[0].Int()),
+			}, nil
+		},
+	})
+}