builtin functions for division and remainder division

* add `@divTrunc` and `@divFloor` functions
 * add `@rem` and `@mod` functions
 * add compile error for `/` and `%` with signed integers
 * add `.bit_count` for float primitive types

closes #217

src/error.cpp

@@ -23,6 +23,8 @@ const char *err_str(int err) {
         case ErrorOverflow: return "overflow";
         case ErrorPathAlreadyExists: return "path already exists";
         case ErrorUnexpected: return "unexpected error";
+        case ErrorExactDivRemainder: return "exact division had a remainder";
+        case ErrorNegativeDenominator: return "negative denominator";
     }
     return "(invalid error)";
 }