x/crypto: add support for Tiny Encryption Algorithm (TEA)

TEA is still in use, particularly in old software.

The algorithm is described at,
http://www.cix.co.uk/~klockstone/tea.pdf
http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm#Reference_code

Reduced-round variations of TEA are annoyingly common, like,
http://daniel.haxx.se/sansa/mi4code.html

Change-Id: I28d7d584d398e3a96371f344624dc60dec75aea3
Reviewed-on: https://go-review.googlesource.com/10825
Reviewed-by: Adam Langley <agl@golang.org>