This implements a minimal wast testing harness in tests/wast.rs, which
runs the wast tests under tests/wast.
It also adds tests for trapping in a variety of ways, and fixes several
bugs exposed by those tests.
* Implement wasm trap handlers.
This adds signal handlers based on SpiderMonkey's signal-handler code.
The functionality for looking up the trap code and wasm bytecode offset
isn't yet implemented, but this is a start.
I considered rewriting this code in Rust, but decided against it for now
as C++ allows us to talk to the relevant OS APIs more directly.
Fixes#15.
* Compile with -std=c++11.
* Refactor InstallState initialization.
* Compile with -fPIC.
* Factor out the code for calling a wasm function with a given index.
* Fix unclear wording in a comment.
