more intuitive left shift and right shift operators

Before:
 * << is left shift, not allowed to shift 1 bits out
 * <<% is left shift, allowed to shift 1 bits out
 * >> is right shift, allowed to shift 1 bits out

After:
 * << is left shift, allowed to shift 1 bits out
 * >> is right shift, allowed to shift 1 bits out
 * @shlExact is left shift, not allowed to shift 1 bits out
 * @shrExact is right shift, not allowed to shift 1 bits out

Closes #413

src/error.hpp

@@ -25,6 +25,7 @@ enum Error {
     ErrorUnexpected,
     ErrorExactDivRemainder,
     ErrorNegativeDenominator,
+    ErrorShiftedOutOneBits,
 };
 
 const char *err_str(int err);