Optimize some hot paths to speed up formatting.

[belav]

A single method in SyntaxNodeComparer was resulting in 10% of the execution time by using a Where on a SyntaxTriviaList before passing it to another method.
Optimizing that to not use Where should make it much more efficient.

The IndentBuilder is another hot path that is doing a bunch of extra work related to dedent, even though CSharpier doesn't support dedenting.

Doc.Concat is unnecessarly creating List when it could use Doc[] or List as IList

A number of Doc's can be a single instance instead of being created over and over.