Optimize basic fexpr functions

[fredrik-johansson]

• Basic traversal functions should be implemented iteratively rather than recursively so that deeply nested expressions don't risk causing stack overflow
• Transformations such as replace could be implemented in a streaming style to avoid recursive copies
• Efficient in-place aliased operations
• Avoid temporaries in big integer operations
• Other todos in the code

[fredrik-johansson]

Also investigate savings in the data representation. For example, calls with many arguments currently build an index. This allows O(1) random access, but increases space and notably prevents gradually appending arguments to an existing function call (since the index has to be expanded periodically).