Merge pull request pingcap#10 from gregwebs/docs-deprecation

update docs around deprecation

Merge pull request pingcap#9 from pingcap/reduce-alloc-print-stack

reduce allocations when printing stack traces (pkg#149)

add NewStack(), which generates a stack trace

release errors 0.8.0

release errors version 0.7.1

errors version 0.7.0

errors version 0.6.0

errors version 0.5.1

errors version 0.5.0

errors release 0.4.0