Add user data to spans.

Separate internal plain Paragraph/ParagraphLayout.

Do not re-export through the Plain module.

Fix style: external vs internal imports.

Fix style: white space around operators.

Fix style: white space in exports and imports.

Remove (~) operator from ParagraphConstruction.

The tilde operator was causing compilation errors with older versions
of GHC, possibly due to a conflict with the lazy pattern match syntax.

It has been replaced by a pure Haskell alternative that uses function
application.

Make SpanOptions a separate type.

Expose ParagraphConstruction.

Move non-public modules into Internal namespace.

Make ParagraphConstruction more efficient.

Remove I8/Int distinction.

The distinction was meant to provide additional type safety, but
inconsistency with other used libraries required too many workarounds
for this to be beneficial.

Clean up ParagraphConstruction.

Use infix operators for constructing test data.