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/signalhandlers/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.0)
+project(SignalHandlers CXX)
+
+set(CMAKE_CXX_FLAGS "-std=c++11 -fno-exceptions -fno-rtti -fPIC")
+
+add_library(SignalHandlers STATIC SignalHandlers.cpp)
+
+install(TARGETS SignalHandlers DESTINATION .)