Add JSON.json(Vector{UInt8}, x) to write JSON to a byte vector

[nhz2]

The new method returns the output as a Vector{UInt8} rather than a String. Getting bytes from Vector{UInt8}(JSON.json(x)) copies the whole output; the new method does not. On Julia 1.13 with a small String => String Dict:

julia> x = Dict("a" => "b", "c" => "d");

julia> @btime Vector{UInt8}(JSON.json($x));
  193.190 ns (10 allocations: 896 bytes)

julia> @btime (io = IOBuffer(); JSON.json(io, $x); take!(io));
  196.597 ns (11 allocations: 976 bytes)

julia> @btime JSON.json(Vector{UInt8}, $x);
  156.349 ns (7 allocations: 784 bytes)

Option construction and validation shared by the json entry points now lives in writeoptions.

This changes the behavior of JSON.json(Vector{UInt8}, x) calls. Previously, an integer x hit the pre-1.0 json(x, indent) compat path and serialized the type itself:

julia> JSON.json(Vector{UInt8}, 1)
"\"Vector{UInt8}\""

It now returns the bytes of 1. Any other x previously threw an AssertionError ("filename must be a string").

Question: Is this behavior change breaking, and if so should this method instead be a new function?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.12%. Comparing base (a69c082) to head (d53dfb1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
+ Coverage   92.09%   92.12%   +0.03%     
==========================================
  Files           7        7              
  Lines        1784     1791       +7     
==========================================
+ Hits         1643     1650       +7     
  Misses        141      141              

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[quinnj]

Yeah, this sounds fine to me.