bcs: clean deps, reduce CI clean build time (#14)

* deps: use serde_core directly

`bcs` only uses proc-macro serde derives in tests. We can reduce compile
times by using `serde_core` directly, which only exports the core serde
traits.

* deps: remove thiserror, impl Error manually

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
```

* msrv: 1.56 -> 1.61, fix CI

In theory we should be able to support an MSRV of `1.56`. Unfortunately this
entry in `serde_core`'s Cargo.toml seems to confuse Cargo into adding
`serde_derive` into the lock file, which only has a compatible MSRV of `1.61`.

```toml
[target.'cfg(any())'.dependencies]
serde_derive = { version = "=1.0.220", path = "../serde_derive" }
```

Author: phlip9

.github/workflows/ci.yml

@@ -13,13 +13,8 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-
-      - name: Install Rust toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          components: rustfmt, clippy
+      - uses: actions/checkout@v6
+      - run: rustup component add rustfmt clippy
 
       - name: Lint
         run: |
@@ -35,10 +30,5 @@ jobs:
   minimum-supported-rust-version:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          # Dependencies may require Rust 2021.
-          toolchain: 1.56.0
-          override: true
-      - run: cargo check
+      - uses: actions/checkout@v6
+      - run: cargo +1.61.0 check --locked --lib

.gitignore

@@ -1,2 +1 @@
 /target
-Cargo.lock