Don't leak uninferred variables into type errors

[Marwes]

Sometimes errors can contain types that have unbound type variables which are represented by integers. These variables should instead be replaced by generalized variables (the type variables which are represented by starting with a lowercase letter in the source).

123 -> a -> Option Int

==>

a1 -> a -> Option Int

https://gitter.im/gluon-lang/gluon?at=592fd309631b8e4e61331814

Generalize here

gluon/check/src/typecheck.rs

Lines 439 to 458 in 5f3df1a

	if self.errors.has_errors() {
	Err(mem::replace(&mut self.errors, Errors::new()))
	} else {
	match ::rename::rename(&mut self.symbols, &self.environment, expr) {
	Ok(()) => {
	debug!("Typecheck result: {}", typ);
	Ok(typ)
	}
	Err(errors) => {
	for Spanned { span, value } in errors {
	self.errors
	.push(Spanned {
	span: span,
	value: value.into(),
	});
	}
	Err(mem::replace(&mut self.errors, Errors::new()))
	}
	}
	}