Reduce the cost of formatting

[labbott]

Text formatting is very expensive. With a few changes we can save a good chunk of text:

Before:

$ arm-none-eabi-size target/thumbv7em-none-eabihf/release/demo
   text	   data	    bss	    dec	    hex	filename
  26284	      0	     48	  26332	   66dc	target/thumbv7em-none-eabihf/release/demo`

After:

$ arm-none-eabi-size target/thumbv7em-none-eabihf/release/demo
   text	   data	    bss	    dec	    hex	filename
  16392	      0	     48	  16440	   4038	target/thumbv7em-none-eabihf/release/demo

This doesn't actually change anything to use ufmt but it sets up a few things so we can eventually convert klog over to something that uses ufmt

bloat output afterwards:

File  .text    Size             Crate Name
0.9%  25.4%  3.5KiB              kern syscall_entry
0.2%   5.5%    770B               std core::fmt::Formatter::pad
0.2%   4.3%    602B               std core::fmt::Formatter::pad_integral
0.1%   3.8%    540B              kern kern::startup::safe_start_kernel
0.1%   3.6%    504B              kern kern::syscalls::deliver
0.1%   3.2%    456B               std core::fmt::write
0.1%   3.2%    452B              kern kern::startup::start_kernel
0.1%   2.9%    404B              kern <kern::arch::arm_m::Mmfsr as core::fmt::Debug>::fmt
0.1%   2.7%    384B              kern kern::syscalls::borrow_lease
0.1%   2.3%    328B              kern DefaultHandler
0.1%   1.8%    252B               std core::fmt::num::imp::<impl core::fmt::Display for u32>::fmt
0.1%   1.5%    212B panic_semihosting <&T as core::fmt::Display>::fmt
0.1%   1.5%    210B               std core::fmt::num::<impl core::fmt::LowerHex for i32>::fmt
0.1%   1.5%    208B              kern kern::arch::arm_m::reinitialize
0.0%   1.3%    180B              kern mem_manage_fault
0.0%   1.2%    172B              kern SysTick
0.0%   1.2%    168B              kern kern::umem::safe_copy
0.0%   1.1%    158B               std __aeabi_memset4
0.0%   1.1%    156B              kern kern::arch::arm_m::start_first_task
0.0%   1.1%    156B              kern kern::task::select
1.1%  29.5%  4.1KiB                   And 78 smaller methods. Use -n N to show more.
3.7% 100.0% 13.8KiB                   .text section size, the file size is 369.8KiB

[cbiffle]

These don't look architecture specific, we could probably move them into kern/src/lib.rs.