removed std from serialize implementation in rust Vector type

[lvboudre]

What

In no_std environment:

flatbuffers = { version = "25.9.23",  default-features = false, features = ["serialization"] }

The Vector type implements serde::ser::Serialize's serialize function by returning std::result::Result instead of core::result::Result, which cause the following type error:

--> /usr/local/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/flatbuffers-25.9.23/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]: failed to resolve: use of unresolved module or unlinked crate `std`
   --> /usr/local/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/flatbuffers-25.9.23/src/vector.rs:321:46
    |
321 |     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
    |
321 -     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
321 +     fn serialize<S>(&self, serializer: S) -> result::Result<S::Ok, S::Error>
    |

This pr remove the std::result for core::result to allow serde in no_std environment.

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