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.

tests/shared/host.c

@@ -0,0 +1,17 @@
+#include <dlfcn.h>
+#include <setjmp.h>
+#include <stdio.h>
+static jmp_buf jb;
+void host_print(const char *s) { printf("host_print: %s\n", s); }
+static void onFatal(const char *m) { printf("caught: %s\n", m); longjmp(jb, 1); }
+int main(int argc, char **argv) {
+    void *h = dlopen(argc > 1 ? argv[1] : "./libmod.so", RTLD_NOW | RTLD_LOCAL);
+    if (!h) { printf("%s\n", dlerror()); return 1; }
+    ((void (*)(void (*)(const char *)))dlsym(h, "finch_module_on_fatal"))(onFatal);
+    printf("add %ld\n", ((long (*)(long, long))dlsym(h, "add"))(2, 40));
+    printf("even %d %d\n", ((_Bool (*)(long))dlsym(h, "is_even"))(4), ((_Bool (*)(long))dlsym(h, "is_even"))(5));
+    ((void (*)(const char *))dlsym(h, "hello"))("Rakya");
+    if (!setjmp(jb)) ((long (*)(long))dlsym(h, "crash"))(7);
+    printf("still alive\n");
+    return 0;
+}

tests/shared/host.h

@@ -0,0 +1 @@
+void host_print(const char *s);

tests/shared/mod.fch

@@ -0,0 +1,22 @@
+import "host.h"
+
+fn greet(str who) -> str {
+    return "hello " + who
+}
+
+export fn add(int a, int b) -> int {
+    return a + b
+}
+
+export fn hello(ptr[char] name) {
+    host_print(greet(str(name)))
+}
+
+export fn crash(int i) -> int {
+    xs := [1, 2, 3]
+    return xs[i]
+}
+
+export fn is_even(int n) -> bool {
+    return n % 2 == 0
+}