Restructure string_utils into a library.

[kirkrodrigues]

Description

We are starting to properly modularize CLP into a set of libraries and executables rather than one monolithic source directory. This PR converts the string_utils.* files into a string_utils library.

Some things to note:

• Instead of placing everything in the clp namespace, we are splitting out functionality that is not CLP-specific into its own namespace. This is to:
  • avoid heavily nested namespaces (see TotW 130), and
  • allow other projects to rely on the general functionality from this repo.
    • We will try to avoid conflicts with popular 3rd-party libraries, but (as usual) it will be up to the users of the CLP repo to ensure they are not using a namespace which conflicts with the ones in the CLP repo.
    • We can eventually split libraries into their own repos as necessary.
• The string_utils library doesn't have its CMake install command set up. We will likely address all the install settings in a PR after the complete restructure (these PRs are meant to be iterative given the magnitude of the change we're making).

Validation performed

• Built successfully.
• Unit tests passed.

[davidlion]

Change to verbose include/string_utils/string_utils.hpp.

[haiqi96]

Looks good to me