Concept: use explicit None for empty parameters instead of eg RangeExpr()

[ducky64]

The default provides a concrete default value (typically ideal, eg, RangeExpr.ALL), while an explicit None means not-set.

Helps limit proliferation of internal constructs through the external API.

Likely requires defining RangeLike and friends as Optional, allowing None to be passed through the chain.

Problems:

• No longer provides type information in the object itself
• In some cases, can be solved using an explicit cast-like op at use sites eg RangeExpr(arg), especially within ArgParam
  • This makes sense in Block definitions
  • Shifting the explicit cast into the library definition probably makes sense, to make the API cleaner
• Is this also as clear for port definitions?
  Related? More accurate constructor argument typing #98