Add support for typed map and slices with method calls

[tw1nk]

I noticed that gval didn't support expressions for typed maps and slice method call.

Example map:

type MyMap map[string][]int

func (m MyMap) Sum(key string) int {
    values, ok := m[key]
    if !ok {
        return -1
    }
    sum := 0
    for _, v := range values {
        sum += v
    }
    return sum
}

[generikvault]

thank you