Compiling counted repetitions uses exponential memory

[lambda-fairy]

Currently, Rose compiles repetitions simply by unrolling them. For example, a{10} is compiled as the "match a" instruction repeated 10 times.

This can be dangerous. Consider what would happen if the user added a few more zeroes to the repeat count. Now, rather than allocating 10 instructions, the compiler will allocate 100 or 1000 or 100 000 instead. In fact, this is a case of exponential memory usage, since every extra digit multiplies the code size by 10.

There are two ways to solve this issue:

• re2 -- place a hard limit on memory usage. If the compiler uses more than a specified amount (1 MB) it halts.
• glennsl's engine -- rather than unrolling loops, track the repeat count in an extra register.

I prefer the re2 method, since it doesn't require changing the VM. The majority of repetitions are small anyway, so it's not worth gutting the implementation just to handle a corner case.