Converting from string to []byte makes a copy, as strings are
immutable and slices are not. Creating a byte slice using
AppendFloat() instead of FormatFloat() avoids creating both the string
and the copy.

Split() uses heap storage for the result array: since we never want
more than two parts we can just use IndexByte() to find the split.
And we don't need a check for the case with more than one '.' since
ParseInt() will error.

For performance: it saves one or two allocations every time you hit
this path.