Speed up literal string patterns and fix a bug of unexpected capturing

• Improve string pattern performance

@match str begin
   "This" => ...
end

Now we use === instead of == to make string comparisons at julia v1.1+.

• Avoid to regard symbol nothing as as-pattern:
  A bug occurred at the previous version, where nothings are not treated as literals, so following assertion used to fail.

@assert @match 1 begin
    nothing => false
    _ => true
end # => 

Solved via defining a special pattern before capturing to handle with this.