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.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# finch build --shared: a library a C program loads, calls by name, and that hands runtime
+# errors to the host instead of exiting.
+set -e
+cd "$(dirname "$0")/shared"
+FINCH=${FINCH:-../../build/finch}
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' EXIT
+"$FINCH" build mod.fch --shared -o "$tmp/libmod.so"
+${CC:-cc} host.c -rdynamic -o "$tmp/host" -ldl
+out=$("$tmp/host" "$tmp/libmod.so")
+expected='add 42
+even 1 0
+host_print: hello Rakya
+caught: mod.fch:17: runtime error: index 7 is out of range (the length is 3)
+still alive'
+if [ "$out" = "$expected" ]; then echo "shared library: passed"; else printf 'shared library: FAILED\n%s\n' "$out"; exit 1; fi