Additionally implement HAL 0.2.5 I2C traits, add examples, some improvements

[bjoernQ]

This PR contains the following changes / additions

• implements the HAL 0.2.5 traits for I2C to make it possible to use existing drivers (only I2C for now since this PR already contains a lot of changes) - implemented as outlined in https://github.com/rust-embedded/embedded-hal#adding-support-for-an-embedded-hal--alpha-version-in-a-hal-implementation
• example of using the ssd1306 driver crate
• blinky example
• serial example
• extends the trap frame to include all registers and make the trap frame mutable if needed (used e.g. in my wifi example to make timer-based task switching possible)
• some minor improvements to the interrupt handling
• small workaround in Clock::freeze - when the desired settings match the settings which are present after boot then don't do any changes (works around problems that some users experienced there, also needed by my wifi example)

In future I'd like to add more of the 0.2.x traits and examples but this is already a lot to review.

[luojia65]

Great!

[mkroman]

Awesome!

Regarding the examples - would they maybe be better to put in the sipeed/bl602-rust-guide project?

Also, what are the files in bin/* for?

[bjoernQ]

Thanks for the kind feedback.

My idea regarding the examples is to have it like https://github.com/stm32-rs/stm32f1xx-hal within the hal-repo since it is nice to have the samples always in sync with the code and in future make it easier to provide a working sample together with new functionality. That way a reviewer gets something to test and tinker with - without the need to write own code or checkout an additional example. Also, during development it's easier to check that nothing else broke by introducing changes by just running the samples.

But certainly, a separate standalone guide is very valuable anyway.

Regarding the bin folder:

• for the peripheral interrupts to work we need to use the CLIC interrupt controller, not the CLINT (which is setup by riscv-rt)
• CLIC needs a different alignment for the trap handler so we cannot re-use the trap handler from the runtime crate
• starting with this PR the trap handler is extended to save / restore all registers
• to make it work with stable Rust we cannot use inline-asm (and I think for the trap handler we would even need naked-functions) and need to have the trap handler as a separately compiled assembly file (asm.S) just like riscv-rt
• there are two files in order to support the soft-float and hard-float ABI (to link the wifi blobs we need hard-float ABI since they are compiled for hard-float and since the chip includes an FPU it's good to be able to use it)

[bjoernQ]

Thanks a lot! I really like the idea to rename the assembly file and the blobs. I guess there won't be anything besides the trap handler every so I renamed it to trap.S and prefixed the blobs with trap_