Linker refactoring (#1773)

* Minor code tidying.

* Document that `Linker::iter`'s iteration order is arbitrary.

* Add a few more tests for `wasmtime::Linker`.

* Refactor `Linker::compute_imports`.

 - Extract the error message generation into a separate function.
 - In the error message, sort the candidates.

* Fix a typo in a comment.

* Add `__rtti_base` to the list of allowed but deprecated exports.

* Don't print an Error message when a program exits normally.

* Update comments to reflect the current code.

* Also allow "table" as an exported table, which is used by AssemblyScript.

src/commands/run.rs

@@ -162,8 +162,6 @@ impl RunCommand {
                 // a message and exit.
                 if let Some(trap) = e.downcast_ref::<Trap>() {
                     // Print the error message in the usual way.
-                    eprintln!("Error: {:?}", e);
-
                     if let Some(status) = trap.i32_exit_status() {
                         // On Windows, exit status 3 indicates an abort (see below),
                         // so return 1 indicating a non-zero status to avoid ambiguity.
@@ -173,6 +171,8 @@ impl RunCommand {
                         process::exit(status);
                     }
 
+                    eprintln!("Error: {:?}", e);
+
                     // If the program exited because of a trap, return an error code
                     // to the outside environment indicating a more severe problem
                     // than a simple failure.