Don't use JSON for custom section format

This commit migrates away from using Serde for the custom section in
wasm executables. This is a refactoring of a purely-internal data
structure to `wasm-bindgen` and should have no visible functional change
on users.

The motivation for this commit is two fold:

* First, the compile times using `serde_json` and `serde_derive` for the
  syntax extension isn't the most fun.
* Second, eventually we're going to want to stablize the layout of the
  custom section, and it's highly unlikely to be json!

Primarily, though, the intention of this commit is to improve the
cold-cache compile time of `wasm-bindgen` by ensuring that for new users
this project builds as quickly as possible. By removing some heavyweight
dependencies from the procedural macro, `serde`, `serde_derive`, and
`serde_json`, we're able to get a pretty nice build time improvement for
the `wasm-bindgen` crate itself:

|             | single-core build | parallel build |
|-------------|-------------------|----------------|
| master      |             36.5s |          17.3s |
| this commit |             20.5s |          11.8s |

These are't really end-all-be-all wins but they're much better
especially on the spectrum of weaker CPUs (in theory modeled by the
single-core case showing we have 42% less CPU work in theory).

crates/backend/src/codegen.rs

@@ -1,14 +1,14 @@
 use std::collections::HashSet;
-use std::sync::atomic::{AtomicUsize, Ordering, ATOMIC_USIZE_INIT};
 use std::sync::Mutex;
+use std::sync::atomic::{AtomicUsize, Ordering, ATOMIC_USIZE_INIT};
 
 use proc_macro2::{Ident, Literal, Span, TokenStream};
 use quote::ToTokens;
-use serde_json;
 use shared;
 use syn;
 
 use ast;
+use encode;
 use util::ShortHash;
 use Diagnostic;
 
@@ -87,20 +87,20 @@ impl TryToTokens for ast::Program {
         );
         let generated_static_name = Ident::new(&generated_static_name, Span::call_site());
 
-        let description = serde_json::to_string(&self.shared()?).unwrap();
+        // See comments in `crates/cli-support/src/lib.rs` about what this
+        // `schema_version` is.
+        let prefix_json = format!(r#"{{"schema_version":"{}","version":"{}"}}"#,
+                                  shared::SCHEMA_VERSION,
+                                  shared::version());
+        let mut bytes = Vec::new();
+        bytes.push((prefix_json.len() >> 0) as u8);
+        bytes.push((prefix_json.len() >> 8) as u8);
+        bytes.push((prefix_json.len() >> 16) as u8);
+        bytes.push((prefix_json.len() >> 24) as u8);
+        bytes.extend_from_slice(prefix_json.as_bytes());
+        bytes.extend_from_slice(&encode::encode(self)?);
 
-        // Each JSON blob is prepended with the length of the JSON blob so when
-        // all these sections are concatenated in the final wasm file we know
-        // how to extract all the JSON pieces, so insert the byte length here.
-        // The value is little-endian.
-        let generated_static_length = description.len() + 4;
-        let mut bytes = vec![
-            (description.len() >> 0) as u8,
-            (description.len() >> 8) as u8,
-            (description.len() >> 16) as u8,
-            (description.len() >> 24) as u8,
-        ];
-        bytes.extend_from_slice(description.as_bytes());
+        let generated_static_length = bytes.len();
         let generated_static_value = syn::LitByteStr::new(&bytes, Span::call_site());
 
         (quote! {