Draft: Implement Glow type descriptors

[kwannoel]

In GitLab by @kwanzknoel on Jul 10, 2021, 02:52

[kwannoel]

This is currently on hold. We should spec out Glow's IR(s) first, to figure out the memory representation of Glow.
After that we can come up with a better interface for type descriptors, since there should be type descriptors for memory layout of types as well (boxed or unboxed etc...).