Miscellaneous parser cleanup (#429)

- Give unary `-` and `not` the same precedence as in C++
- Add detail to parse error messages, and make the --trace flag also enable parser debug tracing
- Make any new shift-reduce conflicts into build errors
- Use `%precedence` rather than `%nonassoc` where possible, in order to catch more grammar bugs at build time

executable_semantics/main.cpp

@@ -12,8 +12,6 @@
 #include "llvm/Support/CommandLine.h"
 
 int main(int argc, char* argv[]) {
-  // yydebug = 1;
-
   using llvm::cl::desc;
   using llvm::cl::opt;
   opt<bool> trace_option("trace", desc("Enable tracing"));