Fix some glitches regarding INTERN/FIND-SYMBOL and multiple threads.

- In a PACKAGE-HASHTABLE, store the hash vector in the last cell
  of the symbol vector so that both can be changed in one write.

- When resizing, fill a temporary vector with existing symbols,
  then install it so that symbols don't seem to disappear and reappear.

- Similary in EXPORT perform ADD-SYMBOL before NUKE-SYMBOL.
Point out possible improvement to SXHASH
Tests: don't test for package documentation without SB-DOC.

Some recent changes made the package documentation to become NIL, not
the previous "", which made the assertion to fail.