Improve parsing of scalar literals

* Unify parsing of string literals and scalar literals, to (e.g.) ensure escapes are handled uniformly. Notably, this makes unicode escapes valid in scalar literals.
* Add a variety of custom error messages about specific failure cases of parsing string/scalar literals. For example, if we're expecting a string (e.g. a package name in the header) and the user tried using single quotes, give a clear message about that.
* Fix formatting of unicode escapes (they previously used {}, now correctly use () to match roc strings)

crates/compiler/can/src/expr.rs

@@ -18,7 +18,7 @@ use roc_module::called_via::CalledVia;
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::Symbol;
-use roc_parse::ast::{self, Defs, EscapedChar, StrLiteral};
+use roc_parse::ast::{self, Defs, StrLiteral};
 use roc_parse::pattern::PatternType::*;
 use roc_problem::can::{PrecedenceProblem, Problem, RuntimeError};
 use roc_region::all::{Loc, Region};
@@ -2297,7 +2297,7 @@ fn flatten_str_lines<'a>(
                         );
                     }
                 }
-                EscapedChar(escaped) => buf.push(unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }
@@ -2355,19 +2355,6 @@ fn desugar_str_segments(var_store: &mut VarStore, segments: Vec<StrSegment>) ->
     loc_expr.value
 }
 
-/// Returns the char that would have been originally parsed to
-pub fn unescape_char(escaped: &EscapedChar) -> char {
-    use EscapedChar::*;
-
-    match escaped {
-        Backslash => '\\',
-        Quote => '"',
-        CarriageReturn => '\r',
-        Tab => '\t',
-        Newline => '\n',
-    }
-}
-
 #[derive(Clone, Debug)]
 pub struct Declarations {
     pub declarations: Vec<DeclarationTag>,

crates/compiler/can/src/pattern.rs

@@ -1,6 +1,6 @@
 use crate::annotation::freshen_opaque_def;
 use crate::env::Env;
-use crate::expr::{canonicalize_expr, unescape_char, Expr, IntValue, Output};
+use crate::expr::{canonicalize_expr, Expr, IntValue, Output};
 use crate::num::{
     finish_parsing_base, finish_parsing_float, finish_parsing_num, FloatBound, IntBound, NumBound,
     ParsedNumResult,
@@ -935,7 +935,7 @@ fn flatten_str_lines(lines: &[&[StrSegment<'_>]]) -> Pattern {
                 Interpolated(loc_expr) => {
                     return Pattern::UnsupportedPattern(loc_expr.region);
                 }
-                EscapedChar(escaped) => buf.push(unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }