fix: 1e-1 edge case

the "wrap up any unfinished parsing" section doesn't need to be changed,
since this edge case is only valid when there is a number after the '-'
which there never is in the case of only one extra character

in other words, 1e- is not valid:
`NumberParsing("1e-", ParseFloatError { kind: ExpNoDigits })`

src/nodes/from_str.rs

@@ -112,7 +112,10 @@ impl FromStr for Nodes {
                     }
                 }
                 ParsingState::ReadingNumber(start_index) => {
-                    if !is_number_char(c) {
+                    if !is_number_char(c)
+                        // don't stop reading number on a special case, eg "1e-1"
+                        && !(filtered_string.chars().nth(i - 1) == Some('e') && c == '-')
+                    {
                         nodes.push(Node::Number(
                             Float::parse(&filtered_string[start_index..i])
                                 .map_err(|e| {

src/nodes/tests.rs

@@ -5,6 +5,11 @@ use rug::Float;
 
 use crate::nodes::{Function, Node, Nodes, Operator, PREC};
 
+#[test]
+fn e_edge_case() {
+    assert_eq!(Nodes::from_str("1e-1").unwrap().evaluate().to_f64(), 0.1)
+}
+
 #[test]
 fn implicit_mult() {
     assert_eq!(