chore: remove fuzz tests for Go 1.15 compatibility

chore: add ChinaTimeLocation

chore: fix typos

refactor: add InitChinaLocation(), deprecated: InitCSTLocation()

chore: fix typos

chore: update examples

perf: improve xfile.TailLines()

feat: add xfile.HeadLines() and xfile.TailLines()

chore: add default parameter to LastString() and FirstString()

feat: add LastString() and FirstString()