Add BinaryWriter Span-based APIs

[stephentoub]

Separated out of https://github.com/dotnet/corefx/issues/21281 for tracking purposes.

• Implement in System.Private.CoreLib in coreclr
• Implement in System.Private.CoreLib in corert (if BinaryWriter isn't yet in "shared")
• Expose from System.IO contract in corefx
• Add tests to System.IO tests in corefx

namespace System.IO
{
    public class BinaryWriter
    {
        public virtual void Write(ReadOnlySpan<byte> value);
        public virtual void Write(ReadOnlySpan<char> value);
        …
    }
}

[pjanotti]

@stephentoub I'm closing this per PRs below

dotnet/coreclr#13378 (also in corert)
dotnet/corefx#23253 (tests and ref in corefx)

[gusarov]

any estimates when this will be available in Net Standard?