Improve correctness and function declarations

[swapneils]

Correctness changes:

• join now treats a nil separator as the empty string, rather than the string "nil", conforming to the semantics in the README of not converting nil to "nil".
• containsp now automatically returns nil if either string input is nil.
• alphanump and similar now use the correct regex checks for multi-line strings, rather than accepting an input if it has any line matching their specification.

Optimization changes:

• Multiple non-recursive functions have been inlined
• Multiple functions have ftype declarations
• In local testing, there is a ~0.1s reduction from ~1.05s to ~0.95s in (time (dotimes (i 100) (with-output-to-string (throwaway) (let ((*standard-output* throwaway)) (asdf:test-system :str))))), despite the new tests in this version.
  • Note: Most of the tests use constantp inputs, so performance improvements here derive mainly from the inlining; they do not represent benefits from the ftype declarations in user-code.
  • Note: The comparison was specifically with the contents of Optimize str:repeat for large workloads #128, but that still shows an improvement over the original.

[swapneils]

Needed to convert this setf to a static dataflow so SBCL could type-check this function.

[vindarel]

excellent, thanks, it looks good, though I'll take more time to test in the wild, because merging bad type declarations can be very annoying (and imagine if Quicklisp releases in-between our quick fixes). I encourage anyone to try this PR.