Use quote instead of syntex for Rust code generation

The `syntex` crate is unmaintained. It is slow to build, and additionally it
requires that we pre-process `src/codegen/mod.rs` before we build the `bindgen`
crate.

The `quote` crate provides similar quasi-quoting functionality, is maintained,
and builds faster. It doesn't have a typed API or builders, however; it only
deals with tokens.

Before this commit:

```
$ cargo clean; cargo build
<snip>
    Finished dev [unoptimized + debuginfo] target(s) in 98.75 secs
```

After this commit:

```
$ cargo clean; cargo build
<snip>
    Finished dev [unoptimized + debuginfo] target(s) in 46.26 secs
```

Build time is cut in half! But what about run time?

Before this commit:

```
Generated Stylo bindings in: Duration { secs: 3, nanos: 521105668 }
```

After this commit:

```
Generated Stylo bindings in: Duration { secs: 3, nanos: 548797242 }
```

So it appears to be about 20ms slower at generating Stylo bindings, but I
suspect this is well within the noise.

Finally, this also lets us remove that nasty `mem::transmute` inside
`bindgen::ir::BindgenContext::gen` that was used for the old `syntex`
context. Now `BindgenContext` doesn't have a lifetime parameter either. This
should make it easier to revisit doing our analyses in parallel with `rayon`,
since that context was one of the things that made it hard for `BindgenContext`
to implement `Sync`.

Fixes #925

Cargo.toml

@@ -39,25 +39,18 @@ diff = "0.1"
 clap = "2"
 shlex = "0.1"
 
-[build-dependencies]
-quasi_codegen = "0.32"
-
 [dependencies]
 cexpr = "0.2"
 cfg-if = "0.1.0"
+# This kinda sucks: https://github.com/rust-lang/cargo/issues/1982
+clap = "2"
 clang-sys = { version = "0.19.0", features = ["runtime", "clang_3_9"] }
 lazy_static = "0.2.1"
 peeking_take_while = "0.1.2"
-syntex_syntax = "0.58"
+quote = "0.3.15"
 regex = "0.2"
-# This kinda sucks: https://github.com/rust-lang/cargo/issues/1982
-clap = "2"
 which = "1.0.2"
 
-[dependencies.aster]
-features = ["with-syntex"]
-version = "0.41"
-
 [dependencies.env_logger]
 optional = true
 version = "0.4"
@@ -66,10 +59,6 @@ version = "0.4"
 optional = true
 version = "0.3"
 
-[dependencies.quasi]
-features = ["with-syntex"]
-version = "0.32"
-
 [features]
 default = ["logging"]
 logging = ["env_logger", "log"]

build.rs

@@ -1,20 +1,11 @@
-mod codegen {
-    extern crate quasi_codegen;
+mod target {
     use std::env;
     use std::fs::File;
     use std::io::Write;
     use std::path::{Path, PathBuf};
 
     pub fn main() {
         let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
-        let src = Path::new("src/codegen/mod.rs");
-        let dst = Path::new(&out_dir).join("codegen.rs");
-
-        quasi_codegen::expand(&src, &dst).unwrap();
-        println!("cargo:rerun-if-changed=src/codegen/mod.rs");
-        println!("cargo:rerun-if-changed=src/codegen/error.rs");
-        println!("cargo:rerun-if-changed=src/codegen/helpers.rs");
-        println!("cargo:rerun-if-changed=src/codegen/struct_layout.rs");
 
         let mut dst = File::create(Path::new(&out_dir).join("host-target.txt"))
             .unwrap();
@@ -77,6 +68,6 @@ mod testgen {
 }
 
 fn main() {
-    codegen::main();
+    target::main();
     testgen::main();
 }