Resolve "Prepare 0.2.1 release"

Closes #113

See merge request !77

Cleanup Spack scripts

Resolve "Add support for Spack deployment and installation"

Closes #96

See merge request !75

Resolve "Prepare release 0.2.0"

Closes #112

See merge request !76

Resolve "Use mochi-thallium instead of our custom Margo engine"

This MR removes our in-house C++ engine for Margo and replaces it with 
mochi-thallium. This makes code simpler since the daemon implementation can 
be fully in C++, which prevents having to muck with C types and their 
lifetimes.

Closes #109

See merge request !74

Resolve "Use C++20"

Closes #108

See merge request !73