Remove DataType and consistently use const Type *

[strongoier]

Currently, types in Taichi are created and owned by:

taichi/taichi/ir/type_factory.h

Line 9 in 77ec1a0

class TypeFactory {

A type with same parameters is only created once, so the pointer to it can serve as its unique identifier, which allows trivial equality check.

Some refactoring tasks:

• Because a type should not be modified once creation, const Type * should be used everywhere.
• The following class,

  taichi/taichi/ir/type.h

  Lines 53 to 54 in 77ec1a0

  	// A "Type" handle. This should be removed later.
  	class TI_DLL_EXPORT DataType {

  , is legacy and should be removed.

[Ris-Bali]

Hey, @lin-hitonami are you still working on this ? If not I would like to have a look at this issue. Thanks

[lin-hitonami]

@Ris-Bali I'm not working on this. Please go ahead if you want to fix the issue.