minor improvements

[Luro02]

I fixed most clippy lints, except the ones from below:

    Checking derive_builder_core v0.8.0 (/media/hdd/home/projects/rust-derive-builder/derive_builder_core)
error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
  --> derive_builder_core/src/bindings.rs:12:22
   |
12 |     pub fn string_ty(&self) -> Path {
   |                      ^^^^^ help: consider passing by value instead: `self`
   |
note: lint level defined here
  --> derive_builder_core/src/lib.rs:25:9
   |
25 | #![deny(warnings, missing_docs)]
   |         ^^^^^^^^
   = note: `#[deny(clippy::trivially_copy_pass_by_ref)]` implied by `#[deny(warnings)]`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:22:22
  |
22|     pub fn result_ty(&self) -> Path {
  |                      ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:32:22
  |
32|     pub fn option_ty(&self) -> Path {
  |                      ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:42:28
  |
42|     pub fn phantom_data_ty(&self) -> Path {
  |                            ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:52:26
  |
52|     pub fn default_trait(&self) -> Path {
  |                          ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:62:24
  |
62|     pub fn clone_trait(&self) -> Path {
  |                        ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:73:23
  |
73|     pub fn into_trait(&self) -> Path {
  |                       ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/bindings.rs:83:27
  |
83|     pub fn try_into_trait(&self) -> Path {
  |                           ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> derive_builder_core/src/options.rs:25:27
  |
25|     pub fn requires_clone(&self) -> bool {
  |                           ^^^^^ help: consider passing by value instead: `self`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

error: aborting due to 9 previous errors

error: could not compile `derive_builder_core`.

To learn more, run the command again with --verbose.

[TedDriggs]

Thank you!