Update no_std support for Rust 2018 Edition.

With Rust 2018 Edition, the `mod std` trick to alias `core` names to
`std` no longer works, so switch to just having the code use `core`
explicitly.

So instead, switch to just using `core::*` for things that in core.
This is more consistent with other Rust no_std code. And it allows
us to enable `no_std` mode unconditionally in the crates that support
it, which makes testing a little easier.

There actually three cases:

 - For things in std and also in core, like `cmp`: Just use them via
   `core::*`.

 - For things in std and also in alloc, like `Vec`: Import alloc as std, as
   use them from std. This allows them to work on both stable (which
   doesn't provide alloc, but we don't support no_std mode anyway) and
   nightly.

 - For HashMap and similar which are not in core or alloc, import them in
   the top-level lib.rs files from either std or the third-party hashmap_core
   crate, and then have the code use super::hashmap_core.

Also, no_std support continues to be "best effort" at this time and not
something most people need to be testing.

lib/serde/Cargo.toml

@@ -5,7 +5,6 @@ authors = ["The Cranelift Project Developers"]
 description = "Serializer/Deserializer for Cranelift IR"
 repository = "https://github.com/CraneStation/cranelift"
 license = "Apache-2.0 WITH LLVM-exception"
-categories = ["no-std"]
 readme = "README.md"
 keywords = ["webassembly", "serde"]
 edition = "2018"
@@ -19,13 +18,8 @@ clap = "2.32.0"
 serde = "1.0.8"
 serde_derive = "1.0.75"
 serde_json = "1.0.26"
-cranelift-codegen = { path = "../codegen", version = "0.26.0", default-features = false }
-cranelift-reader = { path = "../reader", version = "0.26.0", default-features = false }
-
-[features]
-default = ["std"]
-std = ["cranelift-codegen/std"]
-core = ["cranelift-codegen/core"]
+cranelift-codegen = { path = "../codegen", version = "0.26.0" }
+cranelift-reader = { path = "../reader", version = "0.26.0" }
 
 [badges]
 maintenance = { status = "experimental" }