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.

boot/parser.fch

@@ -376,6 +376,7 @@ fn parseStruct(ptr[Parser] p) -> ast.Node {
 
 fn parseFn(ptr[Parser] p) -> ast.Node {
     f := node(p, "fn")
+    f.exported = accept(p, "export")
     expect(p, "fn", "'fn', 'struct' or 'import'")
     f.name = expect(p, "ident", "a function name").text
     if at(p, "[") {