Explain what interning means #419
Description
At the bottom of https://rust-lang.github.io/rustc-guide/the-parser.html, the phrase "interns identifiers" is used. But to me, its meaning wasn't clear on first read-through. I checked in Zulip::/t-compiler/help and got back : "It means that you are not creating a heap allocation for an identifier every time, but instead have a global table where they are stored and deduplicated. You then just need to store the index into the global table whenever you have an identifier" from @oli (oliobk?). It would be helpful to have this explained early on (and maybe a link back to the definition in later uses?)