`run_query` panics on incomplete Prolog queries

[exlee]

Hi,

I wrote pikchr.pl.

Tried to integrate Scryer first, but it panics in a way I couldn't easily fix. It seems that panics come from token parsing (see code below).

scryer-prolog/src/machine/lib_machine/mod.rs

Lines 585 to 593 in 453a88f

	pub fn run_query(&mut self, query: impl Into<String>) -> QueryState<'_> {
	let mut parser = Parser::new(
	Stream::from_owned_string(query.into(), &mut self.machine_st.arena),
	&mut self.machine_st,
	);
	let op_dir = CompositeOpDir::new(&self.indices.op_dir, None);
	let term = parser
	.read_term(&op_dir, Tokens::Default)
	.expect("Failed to parse query");

I get these panics because I'm processing user input in real time, something that's guaranteed to be 95% incomplete.

Not sure how deep the code goes, but worst case scenario this could be fixed with introduction of either Result<_,_> version

pub fn run_query_safe(&mut self, query: impl Into<String>) -> Result<QueryState<'_>>`

or Option<_> version (it looks like the only failure is token parsing, allocation error probably should cause hard panic anyway)

pub fn run_query_safe(&mut self, query: impl Into<String>) -> Option<QueryState<'_>>`

If any solution is acceptable let me know, I can introduce a PR.

[exlee]

After some digging I figured that parser errors are too valuable to throw away and made a PR
(#3236) that adds new run_query_safe with ParserErrors exposed.