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/Cargo.toml

@@ -13,9 +13,16 @@ readme = "README.md"
 cranelift-codegen = "0.25.0"
 cranelift-entity = "0.25.0"
 cranelift-wasm = "0.25.0"
-region = "1.0.0"
 wasmtime-environ = { path = "../environ" }
+region = "1.0.0"
 memmap = "0.7.0"
+lazy_static = "1.2.0"
+libc = "0.2.44"
+
+[build-dependencies]
+cmake = "0.1.35"
+bindgen = "0.43.2"
+regex = "1.0.6"
 
 [features]
 default = ["std"]