New printing support

[ojeda]

Features:

• Adds macros like info! (with newline) and pr_info! (without
  newline) for the usual levels that the kernel supports.

• Includes the "continuation" level.

• Includes automatically prefix with the module name they are called from.

• Includes, hopefully, pretty detailed docs.

• Includes aliases to make it easier to find the equivalent to
  println! for newcomers, since we don't have that one anymore.

Implementation:

• Stack usage should be 1 KiB at most, regardless of the number of
  invocations of the macro in a single function. This should make
  it possible to use even when optimizations are not enabled.
  See code comments for the details.

• The formatting string is compile-time, shared as a static,
  and always the same. We could perhaps pre-generate them
  per-module to avoid another %.*s in printk(), but I don't
  think it is worth the complexity at the moment, and it would
  increase memory usage anyway. This uses the nightly
  const_panic feature -- added to Rust unstable features needed for the kernel #2.

Naming:

• The pr_* names are the same as the kernel ones, and behave
  similarly, i.e. no newline at the end.

• The non-pr_* ones are the same but without the prefix.
  They look better (shorter, no underscore), have the newline like
  println!, and they are what people should almost always use except
  if they need continuation. And they look pretty similar to the Rust
  log crate ones, too!

• I think we can afford these short names (in the prelude, even!) since
  this is new code and nobody should be defining macros in the kernel
  with such short names, ever!

Future:

• I will open an issue with a few things that can be added on top,
  similar to the kernel parameters feature. Done: Complete printing support #122.

• When we have tidy support (or custom clippy lints), we can add
  one to find the pattern:

  pr_*("...\n", ...);
  

  and suggest the non-prefixed, non-newline version instead:

  *("...", "...");
  

Signed-off-by: Miguel Ojeda [email protected]

[ojeda]

• Suggestions from code review.
• Factorize the generate()s by using the same length for the CONT macro too.
• More comprehensive tests to ensure we test the different macro cases.
• Added rust_print.rs sample, updated CI (now we don't need to sed the module parameters tests since the information we need is printed now thanks to the module name being prefixed), etc.
• Removed the devel! macros because there is no easy way that I know of to set a cfg option within Rust code itself (i.e. the C macro is intended for quick debugging where you can write #define DEBUG temporarily).

[ojeda]

A few trivial doc improvements.