Fix serialize feature in rust crate flatbuffers

[alexanderkjall]

'cargo build --no-default-features --features serialize' fails due to std not being available.

Without this patch:

$ cargo build --no-default-features --features serialize
   Compiling flatbuffers v25.12.19 (/tmp/r/flatbuffers-25.12.19)
error[E0433]: cannot find module or crate `std` in this scope
   --> src/array.rs:152:46
    |
152 |     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
    |                                              ^^^ use of unresolved module or unlinked crate `std`
    |
    = help: if you wanted to use a crate named `std`, use `cargo add std` to add it to your `Cargo.toml`
help: consider importing this module
    |
 17 + use core::result;
    |
help: if you import `result`, refer to it directly
    |
152 -     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
152 +     fn serialize<S>(&self, serializer: S) -> result::Result<S::Ok, S::Error>
    |

error[E0433]: cannot find module or crate `std` in this scope
   --> src/vector.rs:326:46
    |
326 |     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
    |                                              ^^^ use of unresolved module or unlinked crate `std`
    |
    = help: if you wanted to use a crate named `std`, use `cargo add std` to add it to your `Cargo.toml`
help: consider importing this module
    |
 17 + use core::result;
    |
help: if you import `result`, refer to it directly
    |
326 -     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
326 +     fn serialize<S>(&self, serializer: S) -> result::Result<S::Ok, S::Error>
    |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `flatbuffers` (lib) due to 2 previous errors

Also some follow-up test fixes to make sure 'cargo build --no-default-features --features serialize' works.