SourceAccessor::readFile should be streaming in constant memory

[roberth]

Describe the bug

This reads a whole file into memory.

void SourceAccessor::readFile(
    const CanonPath & path,
    Sink & sink,
    std::function<void(uint64_t)> sizeCallback)
{
    std::string s = readFile(path);
    sizeCallback(s.size());
    sink(s);
}

Steps To Reproduce

Expected behavior

Multiple sink(chunk) calls.

nix-env --version output

Additional context

Priorities

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