Numerous changes

[tolvanea]

Hi, I continued the project a bit for my own purposes. I'm sharing it here, if it has any use. This pull request is just a information that it exists. If my code is useful, then great, but if not, then feel free to ignore it. I also have no prior experience on git culture nor do I have done pull requests, so I probably do it quite wrong here.

Anyways, here's list of main modifications

• Support for models 3.0, 3.1, 3.2, 3.5, and 3.6. This is theoretical as I tested it only on 3.6.
• Support for hardware floating points on 3.5 and 3.6.
• Implemented wrapper for GPIO pins and millisecond timer.
• Updated teensy3 libraries and updated to compatibility to rust 1.47.
• Refactored of build.rs
• Enabled Teensy build flags with features
• Tried to make the whole process as automated as possible.

If this pull request gets any green light, there are few things that should be changed in it before merging. (I have not implemented them, as I do not know yet if this pull request will get anywhere.) Things that should be changed in my code are at least:

• Remove section "Notes about this fork" from readme
• Update and merge teensy3-rs-demo project to mine
• Update readmes' links between those two projects
• Make teensy3-rs regular dependency in teensy3-rs-demo again, instead of submodule that I currently have there.

If this PR gets any change of being merged, then I can fix those few points above. But anyways, is this project worked on any more?