For Review: User-level synchronous invocations and kernel support.

Summary of this Pull Request (PR)

This PR encompasses all my recent work off of @ldierksheide's namespace implementation including an callgate using MPK for user-level synchronous invocations and kernel integration for detecting and handling invocations that are made from userspace. This is only for review at this stage; this PR covers a large portion of the system from low-level hardware support to higher-level library abstractions so I'm sure there is a lot that could be changed/improved.

Intent for your PR

Choose one (Mandatory):

[x] This PR is for a code-review and is intended to get feedback, but not to be pulled yet.
[ ] This PR is mature, and ready to be integrated into the repo.

Reviewers (Mandatory):

@gparmer @WenyuanShao

Code Quality

As part of this pull request, I've considered the following:

Style:

[x] Comments adhere to the Style Guide (SG)
[x] Spacing adhere's to the SG
[x] Naming adhere's to the SG
[x] All other aspects of the SG are adhered to, or exceptions are justified in this pull request
[ ] I have run the auto formatter on my code before submitting this PR (see doc/auto_formatter.md for instructions)

Code Craftsmanship:

[x] I've made an attempt to remove all redundant code
[x] I've considered ways in which my changes might impact existing code, and cleaned it up
[x] I've formatted the code in an effort to make it easier to read (proper error handling, function use, etc...)
[x] I've commented appropriately where code is tricky
[x] I agree that there is no "throw-away" code, and that code in this PR is of high quality

Testing

I've tested the code using the following test programs (provide list here):

[x] ping_pong_pang_shared_vas.toml

gwsystems / composite