Add "nix verify-paths" command

Unlike "nix-store --verify-path", this command verifies signatures in
addition to store path contents, is multi-threaded (especially useful
when verifying binary caches), and has a progress indicator.

Example use:

$ nix verify-paths --store https://cache.nixos.org -r $(type -p thunderbird)
...
[17/132 checked] checking ‘/nix/store/rawakphadqrqxr6zri2rmnxh03gqkrl3-autogen-5.18.6’

src/nix/progress-bar.cc

@@ -0,0 +1,72 @@
+#include "progress-bar.hh"
+
+#include <iostream>
+
+namespace nix {
+
+ProgressBar::ProgressBar()
+{
+    _writeToStderr = [&](const unsigned char * buf, size_t count) {
+        auto state_(state.lock());
+        assert(!state_->done);
+        std::cerr << "\r\e[K" << std::string((const char *) buf, count);
+        render(*state_);
+    };
+}
+
+ProgressBar::~ProgressBar()
+{
+    done();
+}
+
+void ProgressBar::updateStatus(const std::string & s)
+{
+    auto state_(state.lock());
+    assert(!state_->done);
+    state_->status = s;
+    render(*state_);
+}
+
+void ProgressBar::done()
+{
+    auto state_(state.lock());
+    assert(state_->activities.empty());
+    state_->done = true;
+    std::cerr << "\r\e[K";
+    std::cerr.flush();
+    _writeToStderr = decltype(_writeToStderr)();
+}
+
+void ProgressBar::render(State & state_)
+{
+    std::cerr << '\r' << state_.status;
+    if (!state_.activities.empty()) {
+        if (!state_.status.empty()) std::cerr << ' ';
+        std::cerr << *state_.activities.rbegin();
+    }
+    std::cerr << "\e[K";
+    std::cerr.flush();
+}
+
+
+ProgressBar::Activity ProgressBar::startActivity(const FormatOrString & fs)
+{
+    return Activity(*this, fs);
+}
+
+ProgressBar::Activity::Activity(ProgressBar & pb, const FormatOrString & fs)
+    : pb(pb)
+{
+    auto state_(pb.state.lock());
+    state_->activities.push_back(fs.s);
+    it = state_->activities.end(); --it;
+    pb.render(*state_);
+}
+
+ProgressBar::Activity::~Activity()
+{
+    auto state_(pb.state.lock());
+    state_->activities.erase(it);
+}
+
+}