Refactoring lexer to treat all characters as UTF-8

[tamaroning]

Related to #2287

The lexer currently handles ASCII and non-ASCII characters differently, though Rust accepts UTF-8 source.
The main problem in our lexer is mix use of skip_codepoint_input() (1~4 byte skip) and skip_input(int n) (one byte skip) (also, peek_codepont_input() and peek_input(int n)) , which makes difficult to support Unicode such as identifiers, and whitespaces in our lexer simply.

To deal with this problem, we need

• to modify peek_input(int n) and skip_input(int n) to return and skip a UTF-8 character,
  • Refactor lexer to treat all input characters as UTF-8 #2307
• to replace all use of peek_codepoint_input() and skip_codepoint_input() with peek_input and skip_input respectively,
  • Remove unnecessary methods/fields of Rust::Lexer #2347
• to remove get_codepoint_input_length() and current_char32 field in Lexer,
  • Remove unnecessary methods/fields of Rust::Lexer #2347
• to check if the input source is valid as UTF-8,
  • Refactor lexer to treat all input characters as UTF-8 #2307

[tamaroning]

All tasks are completed.