boot: C import (import "x.h", link, C functions, constants, globals, .ptr, ptr(x))

The self-hosted compiler reads headers with boot/cimport.fch and compiles calls to C
functions (variadics, char* as str, C's default promotions), enum and #define
constants, extern variables like stdout, C structs laid out as C laid them (padding
and fixed arrays, by pointer), .ptr of a str or array, ptr(x) and int(ptr). link
"lib" asks pkg-config, link "x.c" compiles the file along.

cimport and c_pointers pass. Not yet: structs by value (the calling convention) and
addr(fn) for C callbacks, which is also what stops it building itself.

boot.sh now says so when stage 2 can't be made and runs the tests on stage 1.

boot/ast.fch

@@ -111,6 +111,9 @@ struct Program {
     str module
     str path
     []str imports   // Finch modules
+    []str cimports  // C headers: import "stdio.h"
+    []str cwhere    // where each was written, for error messages
+    []str links     // link "glfw", link "mine.c"
     []Node structs
     []Node fns
 }