Update teensy4-bsp, teensy4-pins to latest imxrt-rs dependencies

[mciantyre]

This PR updates

• teensy4-bsp to use imxrt-hal 0.5
• teensy4-pins to use imxrt-iomuxc 0.2

It represents a new release of both packages. See the updated documentation for more information. This work will become teensy4-bsp, version 0.4.

The PR closes #95. The BSP uses imxrt-log as its high-speed USB serial logger. LoggingFrontend documentation and in-tree examples demonstrate how to use the logger in hand-written main() and RTIC applications. We no longer need any C code for this BSP, so this commit removes all the C code in this repository.

I haven't ported all hardware examples in this project. I'm encouraging Teensy 4 contributors to direct their hardware example requests / efforts to imxrt-hal. That project now has in-tree hardware examples that run on various boards, including the Teensy 4. By directing your contributions there, you'll help us bring Rust support to more hardware platforms. I'm happy to maintain teensy4-bsp-specific examples in this repository.

[mciantyre]

bors try

[bors[bot]]

try

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