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/ast.fch

@@ -102,6 +102,7 @@ struct Node {
     Type ty       // declared type (decl, param, field, fn result)
     bool flag     // decl: has a type; or: the right side is a { block }
     bool fallible // fn: -> T!  or  -> !
+    bool exported // export fn: a C function of a library (finch build --shared)
     []Node kids
     []str names   // named arguments: Point(x: 1)
     int line