Use CString::new instead of CString::from_str to init chat template

[AsbjornOlling]

Not sure what happened here but:

cargo build fails for me on the current main branch. It seems like CString doesn't implement from_str()

error[E0599]: no function or associated item named `from_str` found for struct `CString` in the current scope
   --> llama-cpp-2/src/model.rs:50:26
    |
50  |         Ok(Self(CString::from_str(template)?))
    |                          ^^^^^^^^ function or associated item not found in `CString`
    |

This PR just changes it to CString::new() which seems to have the same type signature and behavior.

I guess it could be a thing about rust std versions?

My rustup show says the following, so it doesn't seem like I'm running anything unusual.

Default host: x86_64-unknown-linux-gnu
rustup home:  /home/asbjorn/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu (default)
nightly-x86_64-unknown-linux-gnu

installed targets for active toolchain
--------------------------------------

aarch64-linux-android
x86_64-pc-windows-gnu
x86_64-unknown-linux-gnu

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (default)
rustc 1.85.0 (4d91de4e4 2025-02-17)

I can't really explain why this works in your CI, but not on my machine- but it seems like a pretty uncontroversial change.

[vlovich]

That seems really really strange. You might want to see if you can write a reduced test case as it seem like it would be some kind of bug with Rust stdlib. The change looks fine to me - those two functions are aliases for each other.

[AsbjornOlling]

Something must have been wrong with my environment. I can't have been running rustc 1.85.0.

I've tested more closely with specific versions, and found that CString::from_str works in 1.84.0 and later.

It's pretty clearly visible in the changelog as well: https://releases.rs/docs/1.84.0/

Anyway.. this PR just makes llama-cpp-rs work with rustc versions older than January 9th of this year. A small improvement, but an improvement regardless 😅