Library should publicly export heapless

[ryan-summers]

Currently, if the user application and serde-json-core are using different versions of heapless, the compiler may throw strange errors:

(Example here, app uses heapless 0.6.0, serde-json-core 0.2.0)

error[E0308]: mismatched types
  --> src\net\router.rs:54:24
   |
54 |         self.message = serde_json_core::to_vec(&response).unwrap();
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `heapless::Vec`, found struct `heapless::vec::Vec`
   |
   = note: expected struct `heapless::Vec<u8, UInt<UInt<UInt<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>, B0>, B0>, B0>>`
              found struct `heapless::vec::Vec<u8, _>`
   = note: perhaps two different versions of crate `heapless` are being used?

We should likely be publicly using the heapless types here to allow external users to sync up exactly with the version of heapless the library is using.

[ryan-summers]

Also, the heapless version we are depending on here is not recommended for further use due to a security vulnerability found. We should likely update the heapless version as well.