`incremental` option to `parse!()` + callable Expr compat
Here I've made two changes to make JuliaSyntax easier to integrate into
Base without requiring the existing hook mechanism.
The `incremental` option has been added to `parse!()` moving
flisp-compatible whitespace handling (eg, parsing a statement consumes
up to the next newline) out of the hook and into the main `parse!()` API
function.
The logic for constructing `Expr(:incomplete)` and appropriate
`Expr(:error)` for flisp compatibility is now callable separately from
the hook.
