Marshalling TimeSpan, Guid, BigInt

[jasongin]

Fixes: #66
Fixes: #106

• Marshal .NET Guid and TimeSpan to/from JS string.
  • The Guid string format is compatible with the popular JS uuid package, but there is no dependency on that package, and it mostly operates on string values anyway.
  • A runtime exception with descriptive error message is thrown if the string cannot be parsed when marshalling from JS string to a .NET value.
• Marshal .NET BigInteger to/from JS bigint.
• Update the type-definitions generator accordingly.