* The `codegen_nav`, `codegen_func`, `codegen_type` tasks are renamed to
  `link_nav`, `link_func`, and `link_type`, to more accurately reflect
  their purpose of sending data to the *linker*. Currently, `link_func`
  remains responsible for codegen; this will change in an upcoming
  commit.

* Don't go on a pointless detour through `PerThread` when linking ZCU
  functions/`Nav`s; so, the `linkerUpdateNav` etc logic now lives in
  `link.zig`. Currently, `linkerUpdateFunc` is an exception, because it
  has broader responsibilities including codegen, but this will be
  solved in an upcoming commit.