Reduce compilation memory consumption

[robinhundt]

sine-fdn/polytune#176 shows that just the compilation can already require a significant amount of peak memory for large circuits.

I think some of the memory is required for optimizations done during CircuitBuilder::build(). If that is the case, we could have an option of compiling without optimizations to reduce memory consumption.

[robinhundt]

We reduced peak memory consumption by quite a bit in #236 and #241. The new register circuit representation added in #232 also decreases the size of the resulting circuit by a bit, but requires more memory during compilation.

There are more involved options we could look at to further reduce the memory consumption, so this issue stays open for the moment.