ideas: single value variants of MapContainsValue(s) helpers

[shoenig]

Basically just single Value versions of each of

462:func MapContainsValues[M ~map[K]V, K comparable, V any](t T, m M, vals []V, settings ...Setting) {
474:func MapContainsValuesFunc[M ~map[K]V, K comparable, V any](t T, m M, vals []V, eq func(V, V) bool, settings ...Setting) {
486:func MapContainsValuesEqual[M ~map[K]V, K comparable, V interfaces.EqualFunc[V]](t T, m M, vals []V, settings ...Setting) {

so that we don't need to pass in a slice of values if we're just looking for one. And fix the function signature for the Func variant like on #134