- #3918
- precompute size of `StringBuilder` in `ExpressionHelper`
- reduce `string` allocations in `ViewDataEvaluator`
- also get rid of `Enumeration` state machines
- reduce the size of a few objects; use more expression-valued properties
- e.g. don't store `_modelType` in `ModelExplorer`
- add `EmptyArray<TElement>`; make empty arrays consistently `static`
- avoid `string.Split()` in HTML and tag helpers
nits:
- make `ExpressionHelperTest` tests more stringent
- correct `Message` for an `ArgumentNullException`
- remove excess `using`s in classes I touched (but often ended up leaving otherwise unchanged)
- improve doc comments
- remove `ToString()` call on a `string`
- avoid encoding `string.Empty`
- fix test file name
- remove useless variables
- correct spelling
- improve whitespace
