blockstore/bincode de/serialization refactoring

[dnut]

Address these comments from the blockstore db pr:

#194 (comment)
any reason we have this as a struct other than to group together a few bincode helper functions? - would probably be better to just have the functions (and no wrapper struct) -- also, serializeAlloc is the same as bincode.writeAlloc

#194 (comment)
should just call bincode.readFromSlice instead of this wrapper function

...

if you would want to rename bincode.readFromSlice to bincode.deserialize we can always type declare it as an import to be called deserialize in the files it is used (pub const deserialize = bincode.readFromSlice) and add a TODO: remove after renaming bincode methods throughout codebase

#194 (comment)
this can probably be a bincode helper function - could see it being useful elsewhere too

My thought is basically that I like the isolated namespace for organization, imports, and as a lightweight abstraction, but it's not really that important. This code is about to change soon for other reasons anyway, so we can revisit it at that point.