Use visibility, add postfieldsize, and add stress test

* src/Makefile.am: use visibility
* src/curl.scm.in: add bytevector opt
* src/func.c: (cl_easy_setopt): add postfieldsize to handle
* src/func.h: use visibility
* src/main.c: use visibility
* src/type.c: add postfieldsize to handle
* src/type.h: use visibility, add postfieldsize to handle
* test/Makefile.am: move lib, add stress test
* test/lib.scm: move lib
* test/setopt.scm: renamed from test/t001_test.test
* test/stress.scm: new test