Made Rfc5322Builder::result(&self) return &str
Support parsing messages with only LFs and not CRs.

Fixes issue where the parser would hang on \n-terminated headers.
Fix crash that occurred when headers contained non-whitespace runs of a certain length.

emit_folded() was trying to slice a string from a to b where a > b because it assumed that
it would have found a space before reaching the line length limit.
Preserve folding whitespace as specified in 5322.

Changed consume_linebreak and consume_folding_whitespace: They now return
an Option<&str> that contains \r|\n|\r\n (for _linebreak) or a string of
whitespace (not including the preceding linebreak) (for _folding_whitespace).
Made emit_folded fold lines using existing whitespace instead of added tabs.

It now also accepts tabs as whitespace.