[Trivial] Some DMD warning fixes

Fix warning: `deprecated conversion from string constant to ‘char*’`
Fix warning: `template.c:2899:48: warning: name lookup of ‘i’ changed [enabled by default]
        template.c:2836:16: warning:   matches this ‘i’ under ISO standard rules [enabled by default]
        template.c:2866:25: warning:   matches this ‘i’ under old rules [enabled by default]`