Return a useful object from `create_app` & co.

[KristofferC]

There is some useful stuff that is now only outputted via log messages. For example, when a precompile execution script is run, PackageCompiler prints:

[ Info: PackageCompiler: Executing /home/kc/JuliaPkgs/PackageCompiler.jl/precompile_example.jl => /tmp/jl_packagecompiler_gtWkhz/jl_cZBXfb

However, there is no good way to get at that log file with the precompile statements without basically copy-pasting it. Instead, create_sysimage could return an object where this stuff is stored. So a potential API for this specific case would be

info = create_sysimage(...)
for data in info.precompile_data
    print(data.precompile_execution_file , " logged precompile statements to ", data.precompile_statements_log)
end