Remove methods `sync()`, `get()`, `get_mut()`, `remove_copy()` of
`SharedTensor` and introduce new set of methods: `read()`, `read_write()`,
`write_only()`, `drop_device()`. Signature of `SharedTensor::new()` has also
changed.

New API has following benefits:
 - limited checks of use of uninitialized memory,
 - better memory tracking: several memories can be simultaneously marked as
   up-to-date, so some synchronization operations might be skipped,
 - backing memory is automatically allocated on the first use and added to
   `SharedTensor`, even if it's immutable. Mutability is required only for
   reshaping, modifying actual data and dropping memories.

Rationale and design decisions are discussed at the corresponding bugtracker
issue.

BREAKING CHANGE: sync and memory management API of `SharedTensor`

CLOSE: autumnai#37

…NGELOG]

REFERENCE: autumnai#37

Implementation of SharedTensor uses `unsafe` to extend lifetime of memory
references that are returned by read/write family of methods. Those tests
verify that attempts to create dangling pointers or otherwise misuse API
fail at compile time.

During refactoring (autumnai#37) several error were upgraded into panics. Those
errors may happen only if internal logic of `SharedTensor` is incorrect
and leads to inconsistent state and broken invariants.

…_CHANGELOG]

REFERENCE: autumnai#37

Since plugin operations rely on `SharedTensor`'s memory access and allocation
API, they need to proxy errors. This commit adds new error enum entry to
plugin::Error, and removes deprecated plugin::Error::MissingMemoryForDevice.
It also adds autoconversion from plugin::Error::SharedTensor for convenient
use of `try!`.

…..>`

Allocation of `SharedTensor` may fail only on OOM, so returning `Result` type
is redundant.