[feature] get map keys by values

[RiedleroD]

builtin function that does the equivalent of this (python) function:

def get_keys_by_value(arr: dict, value) -> list:
    return [k for k, v in arr.items() if v == value]

# defining dictionary
arr = {"a": 2, "b": 2, "c": 4, "e": 2}
# getting keys that point to value 2
twos = get_keys_by_value(arr, 2)
# twos now equals ["a", "b", "e"]

this should probably be type checked, i.e. you can't look for a string in a dictionary that has integers. Also this should probably not be supported for doubles, since there's all sorts of silly bs going on there (e.g. -0 and +0 are not the same value). at the very least it should issue a compile-time warning.

i.e., given arr: Map<mK, mV> and value: V, the following should hold true: mV == V && V != double