Simplify the parsing function `spaces`.
Mnemonic name for an argument of `parse`
Restructure the function `spaces`
Take care of blank lines in `spaces`.
Slightly simplify the code to detect a horiz. rule
Make the same decision in `spaces` and the main loop

This is about collapsing newlines into a single one before indented
code.
Use the type aliases in `Omd_parser`
Merge the two decisions about indented code

Fixes https://github.com/pw374/omd/issues/66