Function extend_string 

pub fn extend_string(val: &str, num_bits: u32) -> String

VCD bit extension. This function extends the given string val to match num_bits by adding leading characters according to VCD rules:

• ‘0’ and ‘1’ extend with ‘0’
• ‘x’ extends with ‘x’
• ‘z’ extends with ‘z’
• other leading characters result in no extension