improve printf performance by passing digit buffer around

[JeffBezanson]

Mostly fixes the regression identified in #30218.
Test script:

using Printf

function printfd(n)
    io = open("/dev/null", "w")
    for i = 1:n
        @printf(io, "%d %d\n", i, i + 1)
    end
    close(io)
end

printfd(10)
@time printfd(10000)

Before:

  0.001573 seconds (14 allocations: 832 bytes)

after:

  0.000897 seconds (14 allocations: 832 bytes)

[vtjnash]

LGTM. I don't think an API change can be back-ported though (perhaps unless no packages have been using or extending this).

[JeffBezanson]

I believe they're all internal functions, so we should aim to include this in 1.1 unless we hit a particularly bad problem.

[jmkuhn]

DecFP.jl extends fix_dec() and ini_dec() to allow @sprintf to work.

cc @stevengj

[KristofferC]

Note that DecFP.jl is already broken on master (#30374)

[JeffBezanson]

It's very easy to fix after this pr. Just accept digits as the last argument instead of using the global.