Exp/fix hashmaps for master

This is a roll-up of 2 changes for compatibility with `rust` as of mozilla/rust@6720977.

This patch:
* replaces the codegen for `IterBytes` with codegen for a `Hash` impl. instead.
* replaces the use of `std::hashmap` with `collections::hashmap`

Summary of relevant rust changes:
`IterBytes` has been replaced by `Hash`. (See mozilla/rust@efaf4db24)
`HashMap` has been moved to `collections::hashmap`. (See mozilla/rust@c250c16)

---

Works well enough to build & run drbawb/rust-story@6d32fd96 on Windows & Linux :thumbsup: