conversion function for basic types

[Yoorkin]

• [x] Char

  • to_uint : char.to_uint() is equivalent to char.to_int().reinterpret_as_uint()

• [x] Byte

  • to_uint64
  • to_double

• [ ] Int64

  • to_uint

• [x] Move UInt::to_byte to builtin package because it's useful in here. Should we also move UInt::to_uint64?

• [x] Int::to_uint64

[qazxcdswe123]

Move UInt::to_byte to builtin package because it's useful in here. Should we also move UInt::to_uint64?

This is in uint pkg because we want to reduce the number of func in builtin and UInt::to_byte was removed from builtin as a result. Is this the final decision?

Also byte to double is really weird.

As per discussion offline, some API changes are postponed.

[Lampese]

Float?