Update to the new wasmparser and port to the new readers API.

The new wasmparser API provides dedicated reader types for each section
type, which significantly simplifies the code.

This also changes WasmError::from_binary_reader_error into a From
trait so that we don't have to do .map_err(from_binary_reader_error)
throughout the code.

lib/wasm/src/environ/spec.rs

@@ -3,6 +3,7 @@
 use cranelift_codegen::cursor::FuncCursor;
 use cranelift_codegen::ir::{self, InstBuilder};
 use cranelift_codegen::isa::TargetFrontendConfig;
+use std::convert::From;
 use std::vec::Vec;
 use translation_utils::{
     FuncIndex, Global, GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table, TableIndex,
@@ -62,9 +63,9 @@ pub enum WasmError {
     ImplLimitExceeded,
 }
 
-impl WasmError {
+impl From<BinaryReaderError> for WasmError {
     /// Convert from a `BinaryReaderError` to a `WasmError`.
-    pub fn from_binary_reader_error(e: BinaryReaderError) -> Self {
+    fn from(e: BinaryReaderError) -> Self {
         let BinaryReaderError { message, offset } = e;
         WasmError::InvalidWebAssembly { message, offset }
     }