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/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
+}