Use runtime asserts instead of error returns in desugar.c

The error paths are not tested and are mostly for NULL allocations so
it makes more sense to use runtime asserts for now for simplicity's
sake.

lib/include/desugar.h

@@ -8,6 +8,6 @@
 
 #include "regex.h"
 
-bool desugar_regex(regex_t *regex);
+void desugar_regex(regex_t *regex);
 
 #endif