Internally allow splitting texts at endpoints.

This is required in order to preserve a hard line break at the end of
the input.

Fix style: white space around operators.

Mark some lists as explicitly non-empty.

Remove collapsible spaces at beginning of lines.

Refactor line breaking.

Implement trimming spaces at end of lines.

Move non-public modules into Internal namespace.

Minimise working with Data.Text.Internal.

Implement run splitting.