Common code for MASQ Rust projects

The purpose of sub_lib is to hold the common code for all the shared submodules we use in the node subproject.

It is built as a library, and is not intended as a standalone program.

Historically, each different submodule of node was its own separate project, and sub_lib (sub for Substratum) was the project that held all the common code. Later, the projects (including sub_lib) were all combined into the node project. After that, the need for separate projects (like masq and dns_utility and multinode_integration_tests) reasserted itself on a different level, and we needed another place to put common code. Since by that time our name was MASQ instead of Substratum, all that code went into the masq_lib project.

We think it's unlikely that the current projects in the MASQNode repository will ever be consolidated into one, so both sub_lib and masq_lib will probably continue to exist at their separate levels, although at some point they may take on more descriptive names.

This probably isn't the most interesting place to begin digging into our code; node is a better place to start.

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