Change decimal digits lookup table from static to const

[dtolnay]

Apparently being a static, even with immutable contents, interferes with constant folding. For example buffer.format(i32::MIN) with a static compiles to:

mov rcx, qword ptr [rip + itoa::DEC_DIGITS_LUT@GOTPCREL]
mov rax, rdi
mov rcx, qword ptr [rcx]
movzx edx, word ptr [rcx + 72]
mov word ptr [rdi + 7], dx
mov edx, dword ptr [rcx + 94]
mov esi, edx
shr esi, 16
mov word ptr [rdi + 9], si
mov dword ptr [rdi + 3], edx
mov edx, 11
movzx ecx, word ptr [rcx + 42]
mov word ptr [rdi + 1], cx
mov byte ptr [rdi], 45
ret

and with const:

mov rax, rdi
mov edx, 11
movabs rcx, 3905527098775974194
mov word ptr [rdi + 9], 14388
mov qword ptr [rdi + 1], rcx
mov byte ptr [rdi], 45
ret