Box Block, fn_decl and variant in the AST.

**Note**: this contains the commit from my #10670 PR, don't merge before that. Also, I only tested with those changes, size reductions come from both change sets.

With this, [more enums are shrinked](https://gist.github.com/eddyb/08fef0dfc6ff54e890bc), the most significant one being `ast_node`, from 104 bytes (master) to 96 (#10670) and now to 32 bytes.

My own testcase requires **200MB** less when compiling (not including the other **200MB** gained in #10670), and rustc-stage2 is down by about **130MB**.