XML parsing performance

[jyapayne]

@Araq is there any reason we can't have `--opt:speed` for regular debug builds? 
Using the code from OP yields this:
    
    
    # DEBUG
    nim c -r g.nim
    # Time taken: 14.058434
    
    
    Run
    
    
    # DEBUG with --opt:speed
    nim c -r --opt:speed g.nim
    # Time taken: 3.76424
    
    
    Run