Load string indices with inline asm to save space.

[Dirbaio]

Requires #878 because asm!(... sym ..) breaks if the symbol name has quote characters.

Defmt string indices are 16 bits, which can be loaded with a single movw.
However, the compiler doesn't know that, so it generates movw+movt or ldr rX, [pc, #offs]
because it sees we're loading an address of a symbol, which could be any 32bit value.
This wastes space, so we load the value with asm manually to avoid this.

#[inline(never)]
unsafe fn testfoobar() {
    info!("foo");
}

before:
00027180 <_ZN11application10testfoobar17h819f2778e910636eE>:
   27180: b580         	push	{r7, lr}
   27182: 466f         	mov	r7, sp
   27184: f000 f954    	bl	0x27430 <_defmt_acquire> @ imm = #0x2a8
   27188: 4803         	ldr	r0, [pc, #0xc]          @ 0x27198 <_ZN11application10testfoobar17h819f2778e910636eE+0x18>
   2718a: f000 fa35    	bl	0x275f8 <_ZN5defmt6export6header17hd841c3329e459a6fE> @ imm = #0x46a
   2718e: e8bd 4080    	pop.w	{r7, lr}
   27192: f000 b95b    	b.w	0x2744c <_defmt_release> @ imm = #0x2b6
   27196: bf00         	nop
   27198: 03 00 00 00  	.word	0x00000003

after:
00027180 <_ZN11application10testfoobar17hc6c595839020df66E>:
   27180: b580         	push	{r7, lr}
   27182: 466f         	mov	r7, sp
   27184: f000 f94e    	bl	0x27424 <_defmt_acquire> @ imm = #0x29c
   27188: f240 0003    	movw	r0, #0x3
   2718c: f000 fa34    	bl	0x275f8 <_ZN5defmt6export6header17h79a831a74481cee1E> @ imm = #0x468
   27190: e8bd 4080    	pop.w	{r7, lr}
   27194: f000 b954    	b.w	0x27440 <_defmt_release> @ imm = #0x2a8

[jonathanpallant]

Thanks for this PR - it looks neat. Once we've resolve the question about hex-encoding symbols, I would support merging this.

[jonathanpallant]

(This isn't a breaking change, we just want cargo server-checks to stop complaining about a PR that was committed some time ago)