Random perf improvements

[MihaZupan]

Brings over some of the changes from #499:

• Caching the default pipeline
• Caching for custom writers
• Reduced MarkdownObject size
• Removed most heap allocations from ProcessInlines loop

[MihaZupan]

There is now a split between an exposed Reset and internal ResetInternal.

protected internal void Reset()
{
    // Reset Writer

    ResetInternal();
}

internal void ResetInternal()
{
    // Reset internal state
}

We only use ResetInternal in the library as we can't reset an arbitrary writer.

Would it make sense to change the exposed Reset to not reset the writer and leave that to the caller instead?

[xoofx]

Don't have enough insights on the implication, so I trust your choice 😉