Add `CollectSliceIntoString()` to the collect family, that returns `(result_slice []string, err error)`

[mwsis]

The purpose here is to have a concrete type returned, to simplify (and strengthen) client code.

Consider a custom type Nickname:

type Nickname string

Given an array of nicknames:

nicknames := []Nickname{
  "Goose",
  "Ice Man",
  "Maverick",
}

to obtain a slice of string, one must use CollectSlice() as follows:

a, err := CollectSlice(nicknames, func(input_item any) (any, error) {
  return input_item.(string), nil
})
anys := a.([]any)
strings := make([]string, len(anys))
for ix, a := range anys {
  strings[ix] = a.(string)
}

This is cumbersome, insofar as CollectSlice() returns any, which then has to be subject to a (single-value) type assertion. This in addition to the (single-value) type assertion in the given callback function.

What is required instead is a generic function CollectSliceIntoString():

func CollectSliceIntoStringSlice[T](input_slice []T, func(input_item T*) (string, error)) ([]string, error)

that can then be used without any type assertions in a single-line, as in:

strings := CollectSliceIntoString[Nickname](nicknames, func(input_item Nickname*) (string, error) {
  return string(*input_item), nil
})