lang: core, funcs, types: Add ctx to simple func

Plumb through the standard context.Context so that a function can be
cancelled if someone requests this. It makes it less awkward to write
simple functions that might depend on io or network access.

lang/core/convert/to_float_test.go

@@ -30,13 +30,14 @@
 package convert
 
 import (
+	"context"
 	"testing"
 
 	"github.com/purpleidea/mgmt/lang/types"
 )
 
 func testToFloat(t *testing.T, input int64, expected float64) {
-	got, err := ToFloat([]types.Value{&types.IntValue{V: input}})
+	got, err := ToFloat(context.Background(), []types.Value{&types.IntValue{V: input}})
 	if err != nil {
 		t.Error(err)
 		return