Remove the LLVM/bindgen/cmake dependencies from wasmtime-runtime (#253)

* Remove cmake/bindgen/llvm from wasmtime-runtime

This commit removes the cmake/bindgen dependency (which removes the need
for `llvm-config`) from the `wasmtime-runtime` crate. The C++ code is
instead compiled with the `cc` crate (it's just one file anyway) and the
interface is handwritten since it's quite small anyway.

Some other changes are:

* The `TrapContext` type in C++ was removed since it was unused, and it
  was moved to Rust with a `Cell` on each field.

* Functions between Rust/C++ now return `int` instead of `bool` to make
  them a bit more FFI compatible portably.

* The `jmp_buf` type has a workaround that will be fixed in the next commit.

* Move setjmp/longjmp to C++

This commit moves the definition of setjmp and longjmp into C++. This is
primarily done because it's [debatable whether it's possible to call
`setjmp` from Rust][rfc]. The semantics of `setjmp` are that it returns
twice but LLVM doesn't actually know about this because rustc isn't
telling LLVM this information, so it's unclear whether it can ever be
safe.

Additionally this removes the need for Rust code to know the definition
of `jmp_buf` which is a pretty hairy type to define in Rust across
platforms.

The solution in this commit is to move all setjmp/longjmp code to C++,
and that way we should be able to guarantee that jumps over wasm JIT
code should always go from C++ to C++, removing Rust from the equation
for now from needing to get any fiddly bits working across platforms.
This should overall help it be a bit more portable and also means Rust
doesn't have to know about `jmp_buf` as a type.

The previous `Vec` of `jmp_buf` is now replaced with one thread-local
pointer where previous values are stored on the stack and restored when
the function returns. This is intended to be functionally the same as
the previous implementation.

[rfc]: https://github.com/rust-lang/rfcs/issues/2625

* rustfmt

* Use volatile loads/stores

* Remove mention of cmake from README

README.md

@@ -22,9 +22,11 @@ obtain a full checkout, like this:
 git clone --recurse-submodules https://github.com/CraneStation/wasmtime.git
 ```
 
-To build Wasmtime, install cmake and clang, and then build with cargo, such
-as with "cargo build --release". For information on installing clang, see
-[rust-bindgen's documentation](https://github.com/rust-lang/rust-bindgen/blob/master/book/src/requirements.md).
+To build an optimized version of Wasmtime, use Cargo:
+
+```
+cargo build --release
+```
 
 There are Rust, C, and C++ toolchains that can compile programs with WASI. See
 the [WASI intro][WASI intro] for more information, and the [WASI tutorial][WASI tutorial]