zig build system supports building a library

See #329

Supporting work:
 * move std.cstr.Buffer0 to std.buffer.Buffer
 * add build.zig to example/shared_library/ and add an automated test
   for it
 * add std.list.List.resizeDown
 * improve std.os.makePath
   - no longer recursive
   - takes into account . and ..
 * add std.os.path.isAbsolute
 * add std.os.path.resolve
 * reimplement std.os.path.dirname
   - no longer requires an allocator
   - handles edge cases correctly

example/shared_library/mathtest.zig

@@ -1,7 +1,3 @@
 export fn add(a: i32, b: i32) -> i32 {
     a + b
 }
-
-export fn hang() -> unreachable {
-    while (true) { }
-}