Simply add Lua wrappers for recently introduced prbuf.

Introduce new buffer (in addition to ibuf) - prbuf. "pr" stands for
"partitioned ring-". It save all metadata in the same memory chunk
provided for storage, so it can be completely restored from the 'raw'
memory. API:

```
-- mem is a chunk of raw (char *) memory, of size mem_size.
-- It is used for data storage. Note that available space is of less
-- size due to prbuf metadata overhead.
-- Returns handle to prbuf.
--
require('buffer').prbuf_create(mem, mem_size)

-- mem is a chunk of memory, which contains already created prbuf.
-- It implies that once prbuf_create() was called with the same memory.
-- If mem does not contain valid buffer - raise an appropriate error.
require('buffer').prbuf_open(mem)

-- Returns continuous chunk of memory with given size. May return nil
-- in case if requested chunk is too large. Note that after copying
-- object to returned chunk, it should be committed with prbuf:commit();
-- otherwise :prepare() may return the same chunk twice.
prbuf:prepare(size)

-- Commits the last prepared chunk. Undefined behaviour in case of
-- committing the same chunk twice.
prbuf:commit()

-- Create and return prbuf_iterator. Does not fail. Created iterator
-- points to nowhere - it should be adjusted with :next() call to
-- the first entry.
prbuf:iterator_create()

-- Advance iterator position. Returns prbuf_entry or nil in case
-- iterator has reached the end. Entry consists of two members:
-- size and ptr. The last one is an array of characters of given size.
iterator:next()
```

 Usage examples:

```

local ibuf = buffer.ibuf()
local prbuf_size = 100
local memory = ibuf:alloc(prbuf_size)
local prbuf = buffer.prbuf_create(memory, prbuf_size)

local sample_size = 4
local raw = prbuf:prepare(4)
if raw == nil then
    -- allocation size is too large, try smaller.
end
raw[0] = ...
...
raw[3] = ...
prbuf:commit()
local prbuf_recovered = buffer.prbuf_open(memory)
local iter = prbuf_recovered:iterator_create()
local entry = iter:next()
assert(tonumber(entry.size) == 4)
-- Check values stored in the buffer.
assert(entry.ptr[0] == ...)
entry = iter:next()
-- Our buffer has only one entry.
assert(entry == nil)

```

NO_DOC=<Feature for internal usage>
NO_CHANGELOG=<Feature for internal usage>