Enable performance optimisations in verilator

[hugoferreira]

As per verilator documentation:

-O3 Enables slow optimizations for the code Verilator itself generates (as opposed to "-CFLAGS -O3" which effects the C compiler's optimization. -O3 may improve simulation performance at the cost of compile time.

This PR adds a .with_performance_optimizations(bool) to the builder.

[djg]

Thanks for your PR!