fix: avoid float rounding in Money.to_string

[polvalente]

Big numbers like 123_456_789_123_456_789_123_456_789_123_456_789 suffer from float rounding errors due to the way the conversion in Money.to_string is implemented.

This PR aims to avoid such problems by performing the initial conversion and split using Integer and String functions only.

The following example (included in the new assertions I added) shows the bug.

iex(1)> 123_456_789_123_456_789_123_456_789_123_456_789 |> Money.new(:USD) |> Money.to_string
"$1,234,567,891,234,567,846,791,203,981,885,440.00"
iex(2)> Money.new(123_456_789_123_456_789_123_456_789_123_456_789, :USD) |> Money.to_string == "$1,234,567,891,234,567,891,234,567,891,234,567.89"
false

[Nitrino]

@polvalente Great work. Thanks! 🔥