Umbrella: Model the Drop trait

[nikomatsakis]

This is an umbrella for modeling Drop in formality. There are multiple sub-issues of varying difficulty.

Sub-issues

• Drop: enforce "always applicable" rules #421 — Drop: enforce "always applicable" rules
• Drop: no moving from a struct that implements Drop #422 — Drop: no moving from a struct that implements Drop
• Drop: detect implicit use at scope exit (drop glue) #423 — Drop: detect implicit use at scope exit (drop glue)
• Drop: model #[may_dangle] / dropck eyepatch #424 — Drop: model #[may_dangle] / dropck eyepatch
• Drop: model drop elaboration in codegen #425 — Drop: model drop elaboration in codegen

Context

Modeling Drop correctly requires work across multiple layers:

1. Type checking — enforce that Drop impls are "always applicable"
2. Borrow checking — prevent partial moves from Drop types, insert implicit uses at scope exit
3. Codegen — drop elaboration (what gets dropped, in what order)

The sub-issues are ordered roughly by dependency and difficulty.