Implement wasm trap handlers. (#27)

* 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.

lib/execute/src/lib.rs

@@ -36,13 +36,19 @@ extern crate wasmtime_environ;
 #[cfg(not(feature = "std"))]
 #[macro_use]
 extern crate alloc;
+#[macro_use]
+extern crate lazy_static;
+extern crate libc;
 
 mod execute;
 mod instance;
 mod memory;
+mod signalhandlers;
+mod traphandlers;
 
 pub use execute::{compile_and_link_module, execute, finish_instantiation};
 pub use instance::Instance;
+pub use traphandlers::{call_wasm, LookupCodeSegment, RecordTrap, Unwind};
 
 #[cfg(not(feature = "std"))]
 mod std {