The new script (scripts/nix-flame) is basically a wrapper for creating a
flamegraph of a Nix evaluation (the steps are shown in
`<nix/contrib/stack-collapse.py>`, but are pretty verbose).

The script creates a flame-graph (and displays any evaluation errors in
case of failures) in the SVG-format and optionally opens it using
"$BROWSER" (this was the cheapest approach to open a SVG, if there's a
better approach - feel free to open an issue or file a bug!).

This helper relies on `contrib/stack-collapse.py` which is incorporated from
NixOS/nix (unfortunately this is a contrib-script and not part of any
store path derived from Nix, so we need to bundle it manually). To avoid
license incompliance, a `3rdparty/*` tree was created which contains the
script from Nix including its LICENSE and the notices about our
license situation (MIT for the root, LGPL for the Nix-code) were updated to avoid
any incompatibility.

As this doesn't publish a modified version of NixOS/nix, but basically links
to it (or more specifically a subset of it), referencing the COPYING
file should be fine.