One more: This time, some changes for the uniform vectors (uv) of pmt.
Rationale: std::vector<> always initializes memory.
First, for the uvs, uninitialized may be fine for many cases, otherwise,
initialization should be requested explicitly.
Second, even for scalar types, initialization is expensive, as memset is only
used for byte sized types, otherwise a for loop is used (even when setting to
0).
This patch introduces a small helper class which substitutes std::vector for
the uniform vectors.
This patch has no influence on ABI/API.
It saves about 20% of cycles for test_usrp_inband_tx. (Savings for *_rx are
much smaller, as it only uses uv_u8, not uv_s16, but still significant)