Fix clippy:wrong_self_convention (use-ink#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

src/workspace/profile.rs

@@ -72,7 +72,7 @@ pub enum OptLevel {
 }
 
 impl OptLevel {
-    fn to_toml_value(&self) -> value::Value {
+    fn to_toml_value(self) -> value::Value {
         match self {
             OptLevel::NoOptimizations => 0.into(),
             OptLevel::O1 => 1.into(),
@@ -99,7 +99,7 @@ pub enum Lto {
 }
 
 impl Lto {
-    fn to_toml_value(&self) -> value::Value {
+    fn to_toml_value(self) -> value::Value {
         match self {
             Lto::ThinLocal => false.into(),
             Lto::Fat => "fat".into(),
@@ -118,7 +118,7 @@ pub enum PanicStrategy {
 }
 
 impl PanicStrategy {
-    fn to_toml_value(&self) -> value::Value {
+    fn to_toml_value(self) -> value::Value {
         match self {
             PanicStrategy::Unwind => "unwind".into(),
             PanicStrategy::Abort => "abort".into(),