Steps I take when optimizing the Rust compiler

This commit makes the following writing improvements:

write_to_vec

And the following reading improvements:

• Removes an unnecessary type annotation.
• Fixes a dangerous unchecked slice access. Imagine a slice [0x80] --
  the current code will read past the end of the slice some number of
  bytes. The bounds check at the end will subsequently trigger, unless
  something bad (like a crash) happens first. The cost of doing bounds
  check in the loop body is negligible.
• Avoids a mask on the final byte.

And the following improvements for both reading and writing:

• Changes for to loop for the loops, avoiding an unnecessary

  condition on each iteration. This also removes the need for

  leb128_size .

All of these changes give significant perf wins, up to 5%.

r? @michaelwoerister