[vcd] Add support for MyHDL reporting incorrect types in the header

[ekiwi]

MyHDL will sometimes declare that a variable is of type real in the header but then provide string changes (prefixed by s) in the body of a VCD. This trips up the wellen vcd parser.

Examples:

• pyo3_runtime.PanicException: no entry found for key #89 (comment)
• pyo3_runtime.PanicException: no entry found for key #89 (comment)

Possible solutions:

• pre-scan VCD body to find the type of the first change for every variable (this reduces wellen parsing speed)
  • only perform this pre-scan if the file was generated by MyHDL (check VCD header)
  • OR once parsing fails because of an incorrect type in the header, then do the pre-scan, correct the header and then try parsing again