Add Const kind to AST
Add Const kind to AST

Co-Authored-By: Gabriel Smith <6936c2ba244ab90d0b18d95ac6c5ae264250fe15@users.noreply.github.com>

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust