hashmap: use *_entry APIs to wrap container_of

hashmap: use *_entry APIs to wrap container_of

Using `container_of' can be verbose and choosing names for
intermediate "struct hashmap_entry" pointers is a hard problem.
So introduce "*_entry" APIs inspired by similar linked-list
APIs in the Linux kernel.

Unfortunately, `__typeof__' is not portable C, so we need an
extra parameter to specify the type.

Signed-off-by: Eric Wong <redacted>
Reviewed-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>