Splitted #'ajc-insert-import-at-head-of-source-file-without-confirm
into a few functions.
Removed commented out code.
Added testcase test-ajc-build-plain-method-table-1-check-all-methods.
Made plain method tables be cached.
Made #'ajc-load-tag-file use cache.
Fixed testcase test-ajc-construct-cache-filepath.