Miscellaneous Improvements

[thesps]

This PR brings some miscellaneous quality of life improvements from the performance_estimates branch that are not related to the performance estimates themselves.

• Don't pad trees automatically when unroll is False for xilinxhls backend
• Add Vivado synthesis option to xilinxhls backend
• Add some utilities for model inspection: depth, sparsity
• Make HLS/HDL report reading more robust
• Don't create the vivadohls and vitishls backends, just use xilinxhls. Tool discovery is still automatic (so either Vivado HLS or Vitis HLS will be used automatically). This creation of the backends previously prevents XilinxHLSModel from being pickle-able, which is useful in some cases

[thesps]

Tests pass, including new features. Merging