String to AST

[xealits]

Aha, I just had to install `nimble install compiler`. Otherwise it did not find 
the package `compiler/parser`:
    
    
    $ cat parse_nimcode.nim
    import compiler/parser
    ...
    
    $ nim c -r parse_nimcode.nim
    Hint: used config file '/.../.local/nim/config/nim.cfg' [Conf]
    Hint: used config file '/.../.local/nim/config/config.nims' [Conf]
    ......................................................................
    /.../parse_nimcode.nim(1, 16) Error: cannot open file: compiler/parser
    
    
    Run

But now it complains about a mismatch:
    
    
    import compiler/parser
    
    let
      code_string: string = "echo \"Hello world!\""
    
    let
      code_ast = parseString(code_string)
    
    echo code_ast
    
    
    Run
    
    
    $ nim c -r parse_nimcode.nim
    ...
    /.../parse_nimcode.nim(7, 25) Error: type mismatch
    Expression: parseString(code_string)
      [1] code_string: string
    
    Expected one of (first mismatch at [position]):
    [2] proc parseString(s: string; cache: IdentCache; config: ConfigRef;
                     filename: string = ""; line: int = 0;
                     errorHandler: ErrorHandler = nil): PNode
    
    
    
    Run

It probably needs those `cache: IdentCache` and `config: ConfigRef` in the 
arguments.