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, " ")
}

[moul]

Removing the "must have" requirement since we have alternative ways to call this function. Additionally, it may be more challenging than anticipated because we also need to update the gnoweb help page, as well as possibly the JS and Go clients.