export fn and finch build --shared, in the compiler written in Finch; FINCH_PATH, FINCH_RT, FINCH_CC

export fn name(...) is a C function of a library; build --shared makes lib<name>.so with no main and a
finch_module_on_fatal(handler) the host can take runtime errors over with. Ported from the embed work (the
runtime, its test and the docs are that work's; the compiler side is new).

The driver reads FINCH_PATH (more module folders), FINCH_RT (the runtime's file) and FINCH_CC (the clang).
tests/boot.sh also runs tests/shared.sh with the compiler under test. 41 passed.

.github/workflows/ci.yml

@@ -34,6 +34,8 @@ jobs:
           build/finch version
       - name: Tests
         run: tests/run.sh
+      - name: Shared library
+        run: tests/shared.sh
       - name: Tests under valgrind
         if: matrix.arch == 'x64'
         run: MEMCHECK=1 tests/run.sh