Fix `clippy:wrong_self_convention` (#256)

error: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
  --> src/workspace/profile.rs:75:22
   |
75 |     fn to_toml_value(&self) -> value::Value {
   |                      ^^^^^
   |
   = note: `-D clippy::wrong-self-convention` implied by `-D warnings`
   = help: consider choosing a less ambiguous name
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

error: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
   --> src/workspace/profile.rs:102:22
    |
102 |     fn to_toml_value(&self) -> value::Value {
    |                      ^^^^^
    |
    = help: consider choosing a less ambiguous name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

error: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
   --> src/workspace/profile.rs:121:22
    |
121 |     fn to_toml_value(&self) -> value::Value {
    |                      ^^^^^
    |
    = help: consider choosing a less ambiguous name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention