[Fix Issue54]: Trouble parsing unassigned expressions

[BethanyG]

fixes Issue #54 (we hope)

As a result of this fix, solutions are now being parsed better, and the placeholder assignment has improved.
The following golden tests needed to be updated:

• [example-generator-normalization]
• [plane-tickets]
• [linked-list]
• [nth-prime]
• [palindrome-products]
• [perfect-numbers]
• [pov]
• [pythagorean-triplet]

The following golden test was added to prevent this issue from regressing:

• [example_expression_normalization]

[BethanyG]

Additional Note: Since this code affects mostly hanging unassigned expressions and yield statements, there is probably no need to re-run all solutions. Instead, if there are issues with specific solutions, they can be re-run as needed.