`\` is not properly handled in strings

[KINFOO]

Running the following I get:

M> ast = mlc.luastring_to_ast([[local s ='\ ']])
Evaluation error:
lexer.lua:284: Unknown escape sequence '\ '
stack traceback:
    [C]: in function 'error'
    lexer.lua:284: in function <lexer.lua:279>
    [C]: in function 'gsub'
    lexer.lua:290: in function 'unescape_string'
    lexer.lua:410: in function '?'
    lexer.lua:340: in function 'extract'
    lexer.lua:508: in function 'peek'
    lexer.lua:584: in function 'lineinfo_right'
    gg.lua:604: in function 'parse'
    gg.lua:51: in function <gg.lua:50>
    (tail call): ?
    ...
    gg.lua:51: in function 'chunk'
    compiler/mlc.mlua:133: in function 'f'
    compiler/mlc.mlua:165: in function 'luastring_to_ast'
    stdin:1: in main chunk
    [C]: in function 'xpcall'
    /d/metalua/build/lib/metalua/metaloop.mlua:56: in function 'run'
    compiler/metalua.mlua:251: in function 'main'
    compiler/metalua.mlua:258: in main chunk
    (tail call): ?
    [C]: ?

Whereas, lua interpreter accepts it.

[KINFOO]

@fab13n close this issue if you find the patch good enough.

[wanghc78]

There is another issue may be related to this one. The parser cannot accept a crossing line long string with "".

str = "Hello\
world!"

The parser in koneke Lua eclipse tool has the same problem.

[wanghc78]

Regarding the previous example, the problem happens only in windows platform's line separator. The metalua parser cannot accept the sequence of "\r\n", but can accept "\n".