Add support for TextEncoder#encodeInto

This commit adds support for the recently implemented standard of
[`TextEncoder#encodeInto`][standard]. This new function is a "bring your
own buffer" style function where we can avoid an intermediate allocation
and copy by encoding strings directly into wasm's memory.

Currently we feature-detect whether `encodeInto` exists as it is only
implemented in recent browsers and not in all browsers. Additionally
this commit emits the binding using `encodeInto` by default, but this
requires `realloc` functionality to be exposed by the wasm module.
Measured locally an empty binary which takes `&str` previously took
7.6k, but after this commit takes 8.7k due to the extra code needed for
`realloc`.

[standard]: https://encoding.spec.whatwg.org/#dom-textencoder-encodeinto

Closes #1172

src/lib.rs

@@ -892,7 +892,7 @@ pub mod __rt {
     }
 
     if_std! {
-        use std::alloc::{alloc, dealloc, Layout};
+        use std::alloc::{alloc, dealloc, realloc, Layout};
         use std::mem;
 
         #[no_mangle]
@@ -911,6 +911,27 @@ pub mod __rt {
                 }
             }
 
+            malloc_failure();
+        }
+
+        #[no_mangle]
+        pub extern "C" fn __wbindgen_realloc(ptr: *mut u8, old_size: usize, new_size: usize) -> *mut u8 {
+            let align = mem::align_of::<usize>();
+            debug_assert!(old_size > 0);
+            debug_assert!(new_size > 0);
+            if let Ok(layout) = Layout::from_size_align(old_size, align) {
+                unsafe {
+                    let ptr = realloc(ptr, layout, new_size);
+                    if !ptr.is_null() {
+                        return ptr
+                    }
+                }
+            }
+            malloc_failure();
+        }
+
+        #[cold]
+        fn malloc_failure() -> ! {
             if cfg!(debug_assertions) {
                 super::throw_str("invalid malloc request")
             } else {