fixes dlclark#32 make sure octal parsing only eats chars 0-7

fixes dlclark#24 changes $ behavior in singleline when using RE2 and …

…ECMAScript modes

PCRE and .NET have a different definition of $ than RE2 and ECMAScript
engines in singleline mode.  PCRE defines it as "$ asserts position at the end
of the string, or before the line terminator right at the end of the string (if any)."
This means that a pattern of "^ac$\n" is valid and can match "ac\n" OR "ac".

This behavior is different in RE2 and ECMAScript engines.  For these engines the
pattern "^ac$\n" won't match any inputs in singleline mode because the $ demands the
string ends but the pattern requires an extra \n so they both cannot be true.

The PCRE/.NET behavior feels wrong, but for this project I maintain compatibility with
them in "default" mode.  The other, less suprising behavior is enabled by using either
the RE2 option or the ECMAScript option.

Merge pull request dlclark#29 from eclipseo/fix_conversion_int_to_string

Go 1.15: Convert int to string using rune()

Added RE2 RegexOption that changes parser behavior to include more RE…

…2 formats that the .NET engine does not support. This will allow a smoother transition from the stdlib regexp package.

fix error added during scanning of numeric backreferences with groups

fix issue with groups containing slash constructs like (\c[)

add fuzzing tests for syntax package, fix issues discovered by fuzzing

Add benchmark to match go stdlib

add more detail about code history to ATTRIB file. fixes dlclark#11

prevent panic with multi-byte unicode bmPrefix that partially match. f…

…ixes dlclark#6