Allow using postcard without serde derive

[dtolnay]

Postcard unconditionally enabled serde's "derive" feature (serde_derive dependency) even though the derive is only used for a single type inside postcard (#215). This PR handwrites the Serialize and Deserialize impls for that type and turns off the derive dependency, except for tests.

Before:

postcard v1.1.1
├── cobs v0.2.3
├── heapless v0.7.17
│   ├── hash32 v0.2.1
│   │   └── byteorder v1.5.0
│   ├── serde v1.0.218
│   │   └── serde_derive v1.0.218 (proc-macro)
│   │       ├── proc-macro2 v1.0.94
│   │       │   └── unicode-ident v1.0.18
│   │       ├── quote v1.0.39
│   │       │   └── proc-macro2 v1.0.94 (*)
│   │       └── syn v2.0.99
│   │           ├── proc-macro2 v1.0.94 (*)
│   │           ├── quote v1.0.39 (*)
│   │           └── unicode-ident v1.0.18
│   ├── spin v0.9.8
│   │   └── lock_api v0.4.12
│   │       └── scopeguard v1.2.0
│   │       [build-dependencies]
│   │       └── autocfg v1.4.0
│   └── stable_deref_trait v1.2.0
│   [build-dependencies]
│   └── rustc_version v0.4.1
│       └── semver v1.0.26
└── serde v1.0.218 (*)

After:

postcard v1.1.1
├── cobs v0.2.3
├── heapless v0.7.17
│   ├── hash32 v0.2.1
│   │   └── byteorder v1.5.0
│   ├── serde v1.0.218
│   ├── spin v0.9.8
│   │   └── lock_api v0.4.12
│   │       └── scopeguard v1.2.0
│   │       [build-dependencies]
│   │       └── autocfg v1.4.0
│   └── stable_deref_trait v1.2.0
│   [build-dependencies]
│   └── rustc_version v0.4.1
│       └── semver v1.0.26
└── serde v1.0.218

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