Fix emscripten build failures due to lack of i128 support

[jstarry]

Problem

Bincode only partially relies on whether serde decides to enable i128. And so, building bincode for Emscripten targets is totally broken on rustc >= 1.26.0.

Solution

Remove autocfg and simply rely on serde to determine whether i128 is supported

Background

autocfg checks for i128 support by attempting to compile code to llvm-ir and checking for success or failure. When targeting Emscripten, i128 can still be compiled to llvm-ir and so autocfg believes i128 support should be enabled. Unfortunately, the Emscripten runtime does not support i128 despite the LLVM backend supporting it and so it needs to be explicitly disabled. This is the route that serde takes here: https://github.com/serde-rs/serde/blob/master/serde/build.rs#L56

Since serde does not add the deserialize_u128 method to the serde::Deserializer trait for Emscripten targets, but autocfg believes that 128bit integers are supported and implements the method, I'm seeing this build error:

error[E0407]: method `deserialize_u128` is not a member of trait `serde::Deserializer`
   --> /Users/jstarry/Workspace/bincode/src/de/mod.rs:65:9
    |
65  | /         fn $dser_method<V>(self, visitor: V) -> Result<V::Value>
66  | |             where V: serde::de::Visitor<'de>,
67  | |         {
68  | |             try!(self.read_type::<$ty>());
69  | |             let value = try!(self.reader.$reader_method::<O::Endian>());
70  | |             visitor.$visitor_method(value)
71  | |         }
    | |_________^ not a member of trait `serde::Deserializer`
...
112 |       impl_nums!(u128, deserialize_u128, visit_u128, read_u128);
    |       ---------------------------------------------------------- in this macro invocation

Full CI Failure here: https://travis-ci.com/yewstack/yew/jobs/247388659#L541

[jstarry]

Hey @jdm, could I bother you for a review on this? Thanks :)

[jstarry]

Hi @dtolnay would you mind giving this a review? Yew has been waiting on this change for awhile, thanks!

[dtolnay]

Thanks! This looks good to me.

[dtolnay]

I published 1.2.1 with this fix.

[jstarry]

Thanks!