bcs: clean deps, reduce CI clean build time

[phlip9]

Hey!

I noticed bcs on the critical path of a build.rs script in CI, so I took a pass at fixing that. After some slight refactoring to (1) remove thiserror and (2) use serde_core traits directly, the "1 vCPU in CI" clean build time is down ~5.7x, without many downsides:

Before:

$ cargo tree --edges build,normal
bcs v0.1.6 (/home/phlip9/dev/bcs)
├── serde v1.0.228
│   ├── serde_core v1.0.228
│   └── serde_derive v1.0.228 (proc-macro)
│       ├── proc-macro2 v1.0.106
│       │   └── unicode-ident v1.0.24
│       ├── quote v1.0.45
│       │   └── proc-macro2 v1.0.106 (*)
│       └── syn v2.0.117
│           ├── proc-macro2 v1.0.106 (*)
│           ├── quote v1.0.45 (*)
│           └── unicode-ident v1.0.24
└── thiserror v1.0.69
    └── thiserror-impl v1.0.69 (proc-macro)
        ├── proc-macro2 v1.0.106 (*)
        ├── quote v1.0.45 (*)
        └── syn v2.0.117 (*)

$ rm -rf /tmp/cargo-target && taskset -c 3 cargo build --target-dir /tmp/cargo-target --lib
    Finished dev [unoptimized + debuginfo] target(s) in 7.89s

After:

$ cargo tree --edges build,normal
    Updating crates.io index
bcs v0.1.6 (/home/phlip9/dev/bcs)
└── serde_core v1.0.228

$ rm -rf /tmp/cargo-target && taskset -c 3 cargo build --target-dir /tmp/cargo-target --lib
   Compiling serde_core v1.0.228
   Compiling bcs v0.1.6 (/home/phlip9/dev/bcs)
    Finished dev [unoptimized + debuginfo] target(s) in 1.38s