Fix: Warning: calls to std::mem::drop

[yamakoud]

Issue: #523

What I did:
Fix warning calls to std::mem::drop

Why the problem occurred:
The std::mem::drop function is used to drop (dispose of) a value, taking ownership of that value as its argument.
In this case, a reference (&mut File) was passed to the function.
Since std::mem::drop targets values with ownership and not references, using it on a reference does nothing.
Therefore, this call to drop was unnecessary and the cause of the warning.

Test:

• Build command succeed without any warnings

kyama@Nyanko-Book-Pro shiika % cd lib/skc_rustlib; cargo build; cd ../../
   Compiling libc v0.2.147
   Compiling regex-syntax v0.7.4
   Compiling ppv-lite86 v0.2.17
   Compiling uncased v0.9.9
   Compiling unicode-segmentation v1.10.1
   Compiling plain v0.2.3
   Compiling shiika_ffi_macro v0.1.0 (/Users/kyama/Projects/shiika/lib/shiika_ffi_macro)
   Compiling phf_shared v0.11.2
   Compiling phf_generator v0.11.2
   Compiling phf v0.11.2
   Compiling phf_codegen v0.11.2
   Compiling cc v1.0.82
   Compiling getrandom v0.2.10
   Compiling time v0.1.45
   Compiling rand_core v0.6.4
   Compiling rand_chacha v0.3.1
   Compiling chrono v0.4.26
   Compiling rand v0.8.5
   Compiling cmake v0.1.50
   Compiling regex-automata v0.3.6
   Compiling bdwgc-alloc v0.6.6
   Compiling regex v1.9.3
   Compiling parse-zoneinfo v0.3.0
   Compiling chrono-tz-build v0.0.3
   Compiling chrono-tz v0.6.3
   Compiling skc_rustlib v0.1.0 (/Users/kyama/Projects/shiika/lib/skc_rustlib)
    Finished dev [unoptimized + debuginfo] target(s) in 9.67s
    ```

[yamakoud]

Ready for review

@yhara
Please take a look when you have time.

[yhara]

Therefore, this call to drop was unnecessary and the cause of the warning.

Oh really? Let me explain what I wanted to do.

• In Shiika, opened files should be closed when the File object is GC'ed (otherwise, the file is kept open until the end of the program)
• However Rust does not provide explicit API like std::fs::close(). Instead, the file is closed when the fs::File is dropped
• This std::mem::drop is intended to close the file manually

I thought once I confirmed that this closes the file, but forgot how... I'll try it later.