Simplify global state management in adapter (#13)

* Simplify global state management in adapter

I was looking recently to implement args-related syscalls but that would
require yet-more globals and yet-more state to be managed. Instead of
adding a slew of new globals for this which are all manually kept in
sync I opted to instead redesign how global state is managed in the
adapter.

The previous multiple `global`s are all removed in favor of just one, as
sort of a "tls slot" of sorts. This one remaining slot points to a
one-time-allocated `State` object which internally stores information
like buffer metadata, fd information, etc. Along the way I've also
simplified syscalls with new methods and `?`-using closures.

* Turn off incremental for dev builds

Helps with CGU splitting and ensuring that appropriate code is produced
even without `--release`.

* Review comments

* Add accessors with specific errors
* Update handling of `*_global_ptr`

* Update internal mutability around path buffer

Use an `UnsafeCell` layering to indicate that mutation may happen
through `&T`.

test-programs/macros/build.rs

@@ -9,7 +9,7 @@ fn main() {
 
     let mut components = Vec::new();
 
-    println!("cargo:rerun-if-changed=../../");
+    println!("cargo:rerun-if-changed=../../src");
     let mut cmd = Command::new("cargo");
     cmd.arg("build")
         .arg("--release")