Explore gradual type checking and success typings

[alexrp]

Depends on:

• Implement semantic analysis #10

[alexrp]

celerity/src/semantics/Tree/SemanticTree.xml

Lines 92 to 99 in bf95975

	<!-- Types -->
	<!-- TODO: Should types be represented as nodes or be a higher-level concept like scopes/symbols? -->
	<Type Name="Type" Abstract="true" />
	<Type Name="ReturnType" Abstract="true" />