Perform constant propagation as part of/immediately after parsing the AST

[frebib]

Constant propagation can somewhat/significantly reduce the complexity of the function graph by "merging" vertexes of constant and/or pure functions together.

Some things that could be simplified are:

• Numbers (ints/floats)
• Boolean expressions
• String literals
• (All literal expressions, including the above)
• Identity functions
• Functions that ignore all arguments
• ...