Inline give for Buffer

[antiguru]

This fixes a regression introduced by #426 where BufferCore::give now lacked the inline annotation. In pingpong, this changes the runtime as follows:

Without inline:

target/release/examples/pingpong 1 500000000 -w1  2.45s user 0.00s system 99% cpu 2.446 total

With inline:

target/release/examples/pingpong 1 500000000 -w1  1.75s user 0.00s system 99% cpu 1.755 total

[frankmcsherry]

Looks great and thanks!