feat: add ln function

this one is easier than log or root, because it only takes one arg

src/nodes/eval.rs

@@ -52,6 +52,8 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             Node::Number(n) => match variant {
                 Function::Sqrt => n.sqrt_mut(),
 
+                Function::Ln => n.ln_mut(),
+
                 Function::Sine => n.sin_mut(),
                 Function::ArcSine => n.asin_mut(),
                 Function::Cosine => n.cos_mut(),

src/nodes/mod.rs

@@ -76,6 +76,8 @@ pub enum Operator {
 pub enum Function {
     Sqrt,
 
+    Ln,
+
     Sine,
     ArcSine,
     Cosine,

src/nodes/parse.rs

@@ -37,6 +37,8 @@ impl FromStr for Function {
         match s {
             "sqrt" => Ok(Self::Sqrt),
 
+            "ln" => Ok(Self::Ln),
+
             "sin" => Ok(Self::Sine),
             "asin" => Ok(Self::ArcSine),
             "cos" => Ok(Self::Cosine),