remove allocation in skipchars

[stevengj]

skipchars was using readline(io) to ignore line comments, which allocates a String that is immediately discarded. This PR switches it to copyline(devnull, io, keep=true) instead, exploiting #48273, which shouldn't allocate. (I pass keep=true because it's slightly more efficient in general, since it can skip the CRLF logic.)

(The existing tests cover this change.)

[stevengj]

Unfortunately, despite removing allocations, this actually slows things down for the current version. A quick benchmark reading and discarding all of the lines of HISTORY.md gives:

julia> using_readline(io) = while !eof(io); readline(io); end;

julia> using_copyline(io) = while !eof(io); copyline(devnull, io, keep=true); end;

julia> io = open("HISTORY.md");

julia> @btime using_readline(seekstart($io));
  505.072 μs (5620 allocations: 480.03 KiB)

julia> @btime using_copyline(seekstart($io));
  17.041 ms (0 allocations: 0 bytes)

The basic problem is that copyline(::DevNull, ::IOStream) uses the fallback character-by-character method, which is slow for IOStream. To make this faster we'd need an optimized IOStream method for DevNull. Not sure if it's worth it. 😢

(Note that if someone really wants a faster skipchars, an optimized method for BufferedInputStream is implemented in JuliaIO/BufferedStreams.jl#77.)