Improve performance for 0.27 change on RenderContext

[sunng87]

From 0.27, RenderContext now owns Context. During the render process, we need to clone the RenderContext, so Context and its wrapped data are cloned again and again. This patch made data in Context an Rc, the clone cost will be pretty cheap.

Address issue #166 .

Benchmark:

test large_loop_helper ... bench:   3,242,381 ns/iter (+/- 313,516)

Compared with 0.27:

test large_loop_helper ... bench: 496,376,347 ns/iter (+/- 148,433,745)

0.26:

test large_loop_helper ... bench:   3,148,455 ns/iter (+/- 72,797)