some improvements/optimizations

[agentjill]

some changes on following

• Unnecessary cloning of values removed and replaced with alternative strategy wherever felt necessary.
• Some unnecessary string invocation removed and used string slices to save on some space.
• Added additional definitions to const_str.rs.
• changed code to idiomatically use options and results.
• nested code simplified by adding guarded if
• in cells.rs and rows.rs the logic changed to remove cloning by using take function as it is just changing the key value of a hashmap with itself.
• simplified update_pivot_cache function in src/structs/spreadsheet.rs

[MathNya]

@agentjill Thank you for everything. We will merge them. After adding some fixes, we will release it as version 1.2.0 around the end of this week.