Remove three unwraps from escaped_char

[divergentdave]

I found through fuzzing that the minimized input \d00000 panics upon unwrapping None. This PR adds that input as a test, and refactors escaped_char() to use additional combinators instead of unwrap calls.

[kaj]

Looks good. Thanks!

[connorskees]

@divergentdave you may also want to upstream this issue to dart-sass as it looks like this unexpectedly panics there as well.

With the same input,

Unexpected exception:
RangeError: Invalid value: Not in range 0..1114111, inclusive: 13631488


dart:core                                                  new String.fromCharCode
package:sass/src/parse/parser.dart 462:21                  Parser.escape
package:sass/src/parse/parser.dart 173:18                  Parser.identifier
package:sass/src/parse/stylesheet.dart 455:27              StylesheetParser._variableDeclarationOrInterpolation
package:sass/src/parse/stylesheet.dart 268:35              StylesheetParser._variableDeclarationOrStyleRule
package:sass/src/parse/stylesheet.dart 187:15              StylesheetParser._statement
package:sass/src/parse/stylesheet.dart 89:46               StylesheetParser.parse.<fn>.<fn>
package:sass/src/parse/scss.dart 136:32                    ScssParser.statements
package:sass/src/parse/stylesheet.dart 89:29               StylesheetParser.parse.<fn>
package:sass/src/parse/parser.dart 681:22                  Parser.wrapSpanFormatException
package:sass/src/parse/stylesheet.dart 85:12               StylesheetParser.parse
package:sass/src/ast/sass/statement/stylesheet.dart 88:54  new Stylesheet.parseScss
package:sass/src/ast/sass/statement/stylesheet.dart 66:27  new Stylesheet.parse
package:sass/src/import_cache.dart 193:25                  ImportCache.importCanonical.<fn>
dart:collection                                            _LinkedHashMapMixin.putIfAbsent
package:sass/src/import_cache.dart 188:25                  ImportCache.importCanonical
package:sass/src/compile.dart 56:30                        compile
package:sass/src/executable/compile_stylesheet.dart 91:13  compileStylesheet
\home\travis\build\sass\dart-sass\bin\sass.dart 65:15      main

The relevant code for this panic is here, https://github.com/sass/dart-sass/blob/master/lib/src/parse/parser.dart#L462

It may be clearer to use \110000 rather than \d00000 as the former is the first value (std::char::MAX + 1) to reproduce this issue.