support variadic types as function args in MsgCall

[thehowl]

From discussion in #1610.

Running maxetx call on a variadic function (like the below) will panic claiming that the slice type is not supported.

I think it is reasonable to support variadic types if the underlying types can be easily parsed by the vm keeper.

package variadic

import "strings"

func Echo(vals ...string) string {
	return strings.Join(vals, " ")
}