* Initial implementation of block string literals following lexical_conventions/string_literals.md.
Enabled yyinput() in flex to implement parsing.
Added ParseBlockStringLiteral() helper to handler further transformations such as indenting.
Modified formar_grammar to support single-quoted strings to prevent a failure on lexer.lpp.
* Fixed _find_string_end quote parameter type int -> str.
* Update executable_semantics/syntax/BUILD
Co-authored-by: Geoff Romer <gromer@google.com>
* Addressed code review comments - split table-drived test into individual tests, renamed constants to match style guide.
* Addressed code review comments -- using EXPECT_THAT_EXPECTED() in tests, lexer comments and code cleanup.
Co-authored-by: Geoff Romer <gromer@google.com>
