Fix Calendar.strftime/3 width calculations

[AlexGx]

Preferred formats measured by their iodata element count instead of rendered length

iex> Calendar.strftime(~N[2019-08-15 17:07:57], "%20c")
"0000000002019-08-15 17:07:57" # expected: "02019-08-15 17:07:57"

Unicode values measured in bytes, must in graphemes

iex> Calendar.strftime(%{month: 9}, "%10B", month_names: fn _ -> "вересень" end)
"вересень" # expected: "  вересень"

[josevalim]

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