Undefined Sanitizer found the following error.

```
sudo ./src/bpftrace -e 'BEGIN {@ = 1e30;}'
/home/ubuntu/work/bpftrace/src/utils.cpp:809:10: runtime error: 1e+30 is outside the range of representable values of type 'unsigned long'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/ubuntu/work/bpftrace/src/utils.cpp:809:10 in
Attaching 1 probe...
^C

@: 0
```

To fix this, in the parse_exponent(), check the exponent value and if
it's bigger than uint64_t'x max value, throw exception. The Lexer then
make an error.

Now it becomes

```
% sudo ./src/bpftrace -e 'BEGIN {@ = 1e30;}'
stdin:1:12-16: ERROR: 1e30 is too big for uint64_t
BEGIN {@ = 1e30;}
           ~~~~
stdin:1:12-17: ERROR: syntax error, unexpected ;
BEGIN {@ = 1e30;}
           ~~~~~
```