Problem:
- Many constructors are defined as `{}` rather than using the ` =
default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
instead of requiring the caller to default construct. This violates
the C++ Core Guidelines suggestion to declare single-argument
constructors explicit
(https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).
Solution:
- Change default constructors to use the compiler-provided default
constructor.
- Remove implicit conversion operators from `nullptr_t` and change
usage to enforce type consistency without conversion.
This fixes a bug where lines starting with a space would get parsed as
"level 0" headings - it would not find a "#" and therefore never
increase the level counter (starting at zero), which then would cause
the check for "space after #" pass (again, there is no "#").
Eventually we would get funny results like this:
<h0>[n-1 spaces]oops!</h0>
Also ASSERT(level > 0) in the Heading constructor.
Previously, all Markdown blocks had a virtual parse method which has
been swapped out for a static parse method returning an OwnPtr of
that block's type.
The Text class also now has a static parse method that will return an
Optional<Text>.
