Generalize string types to take as arguments

`asciidoctor_rs::parse` and `asciidoctor_rs::parsing::tokenizer::Tokenizer::new` will take the more generic string types as arguments.

src/lib.rs

@@ -25,7 +25,7 @@ pub(crate) mod utils;
 /// ```rust
 /// let expression = asciimath_rs::parse("sin(2x) + 3".to_string());
 /// ```
-pub fn parse(content: String) -> Expression {
+pub fn parse<S: AsRef<str>>(content: S) -> Expression {
     let mut tokenizer = Tokenizer::new(content);
     let tokens = tokenizer.parse();
     let mut tree_parser = TreeParser::new(tokens);

src/parsing/tokenizer.rs

@@ -19,8 +19,8 @@ pub struct Tokenizer {
 }
 
 impl Tokenizer {
-    pub fn new(text: String) -> Self {
+    pub fn new<S: AsRef<str>>(text: S) -> Self {
-        let mut chars = text.chars().collect::<Vec<char>>();
+        let mut chars = text.as_ref().chars().collect::<Vec<char>>();
         chars.push('\n');
         Self {
             ctm: CharTapeMachine::new(chars),