Error handling and small editor

- Error handling to signal what is preventing evaluation instead of
  program crash
- Small editor feature (allow edit new
line of code to complete the previous ones instead of destroying the
input, if an empty line inserted return the error preventing evaluation
- Auto indentation on multiline input based on depth

Still does not parse multi-statement code
- This is a problem when dealing with macros: does not allow
  expressions like `'()` since the atomic `'` hides the list ().
  Need to chose between:
  - Replace `'...` with `(quote ... )` during tokenization (may be
    hard to implement macros later)
  - Allows multi-statement code (this also allows to execute multiple
    statements when reading a file)

Will probably delete auto-indentation since it breaks code's uniformity
too much

src/step1_read_print.rs

@@ -6,12 +6,15 @@
 
 use crate::printer::pr_str;
 use crate::reader::read_str;
-use crate::types::MalType;
+use crate::types::{MalErr, MalType};
 
 #[allow(non_snake_case)]
 /// Read input and generate an ast
-fn READ(input: &str) -> MalType {
-    read_str(input)
+fn READ(input: &str) -> Result<MalType, (MalErr, usize)> {
+    match read_str(input) {
+        Ok(ast) => Ok(ast),
+        Err((err, depth)) => Err((format!("Unexpected error during READ: {}", err), depth)),
+    }
 }
 
 #[allow(non_snake_case)]
@@ -24,11 +27,11 @@ fn EVAL(ast: MalType) -> MalType {
 #[allow(non_snake_case)]
 /// Print out the result of the evaluation
 fn PRINT(input: MalType) -> String {
-    pr_str(&input)
+    pr_str(&input, true)
 }
 
-pub fn rep(input: &str) -> String {
-    let ast = READ(input);
+pub fn rep(input: &str) -> Result<String, (MalErr, usize)> {
+    let ast = READ(input)?;
     let out = EVAL(ast);
-    PRINT(out /*&result*/)
+    Ok(PRINT(out))
 }