Convert compiler to use Drop trait

[bstrie]

pcwalton asserts that the Drop trait is implemented, though perhaps buggy. Use this to replace the drop block in structs.

[bstrie]

Blocked on #5377, #5378, #5379, #5380, #5382

[catamorphism]

Looks unlikely to get done for 0.6, alas. De-milestoning

[graydon]

@pcwalton is this in-scope or related to what you're doing in 0.7?

[graydon]

assigning bug; change assignment if you disagree

[bstrie]

Drop blocks are gone!