Fix various Clang-Tidy diagnostics

Address most occurrences of:
bugprone-unused-local-non-trivial-variable
bugprone-unhandled-self-assignment
bugprone-misplaced-widening-cast
bugprone-sizeof-expression
performance-for-range-copy
performance-faster-string-find
performance-inefficient-vector-operation
performance-noexcept-swap
misc-unused-using-decls
misc-header-include-cycle
misc-include-cleaner
readability-delete-null-pointer
readability-container-size-empty
readability-redundant-smartptr-get
readability-else-after-return