Use Linker in *.wast testing (#1391)

* Use `Linker` in `*.wast` testing

By default `Linker` disallows shadowing previously defined items, but it
looks like the `*.wast` test suites rely on this so this commit adds a
boolean flag to `Linker` as well indicating whether duplicates are
allowed.

* Review comments

* Add a test with a number of recursive instances

* Deny warnings in doctests

* No tabs

crates/api/src/lib.rs

@@ -7,6 +7,8 @@
 //! itself for consumption from other languages.
 
 #![deny(missing_docs, intra_doc_link_resolution_failure)]
+#![doc(test(attr(deny(warnings))))]
+#![doc(test(attr(allow(dead_code, unused_variables, unused_mut))))]
 
 mod externals;
 mod frame_info;