chore: fix cargo clippy warning

[xixishidibei]

Running cargo clippy reports the following error.

warning: length comparison to zero
   --> src/library/lib.rs:137:12
    |
137 |         if self.libs.len() > 0 {
    |            ^^^^^^^^^^^^^^^^^^^ help: using `!is_empty` is clearer and more explicit: `!self.libs.is_empty()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_zero
    = note: `#[warn(clippy::len_zero)]` on by default

warning: manually reimplementing `div_ceil`
   --> src/library/marshaller.rs:197:23
    |
197 |         let n_bytes = (cnt + self.bit_pos.to_u8() + 7) / 8;
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `(cnt + self.bit_pos.to_u8()).div_ceil(8)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_div_ceil
    = note: `#[warn(clippy::manual_div_ceil)]` on by default

warning: useless conversion to the same type: `core::util::Site<library::lib::LibId>`
   --> src/library/exec.rs:261:40
    |
261 |                     return Jump::Instr(site.into());
    |                                        ^^^^^^^^^^^ help: consider removing `.into()`: `site`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
    = note: `#[warn(clippy::useless_conversion)]` on by default

warning: useless conversion to the same type: `core::util::Site<library::lib::LibId>`
   --> src/library/exec.rs:266:39
    |
266 |                     return Jump::Next(site.into());
    |                                       ^^^^^^^^^^^ help: consider removing `.into()`: `site`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion

warning: `aluvm` (lib) generated 4 warnings (run `cargo clippy --fix --lib -p aluvm` to apply 4 suggestions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 14.40s

This commit resolves the issue.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 57.8%. Comparing base (3e8f8ea) to head (0468ad9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/library/exec.rs	0.0%	2 Missing ⚠️
src/library/lib.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #139     +/-   ##
========================================
- Coverage    57.9%   57.8%   -0.1%     
========================================
  Files          18      18             
  Lines        1317    1309      -8     
========================================
- Hits          763     757      -6     
+ Misses        554     552      -2     

Flag	Coverage Δ
rust	57.8% <25.0%> (-0.1%)	⬇️

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[dr-orlovsky]

ACK 0468ad9

[dr-orlovsky]

Thank you and congrats with the first contribution!