Vectors that can be looked up by the specidied key.
Example
using IndexedVectors
# base data
data = [("a", 100), ("b", 200), ("c", 300)]
# create index by the first item
indexed_data = IndexedVector(data, x -> x[1])
# get by index
data[1] # ("a", 100)
# get by key
data["b"] # ("b", 200)
# slice index
data[1:2] # [("a", 100), ("b", 200)]