Leading / in heading results in error

[bottd]

A heading beginning with / will result in a panic

* /my-heading

[benlubas]

"test.norg" contains:
* /Heading
thread 'main' panicked at src/stage_3.rs:695:43:
called `Result::unwrap()` on an `Err` value: [Simple { span: 2..2, reason: Unexpected, expected: {Some(Special('<')), Some(Special('[')), Some(Special('`')), Some(Special('{'))}, found: None, label: None }]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We unwrap the result of parse_paragraph() which on first glace looks like it always returns Ok but it's got a ? in it, so it can fail.

I've experienced a similar case of this paragraph parsing logic failing. We should obviously go through an remove all the unwrap/expects now that this parser is used in a real plugin that people are using.