Lex: context with &str and lifetime, instead of String and clone

[yuanbohan]

Reference

• calc_manual_lex
• grmtools parsing idioms

[waynexia]

There is a section in grmtool's manual talking about this: https://softdevteam.github.io/grmtools/latest_release/book/parsing_idioms.html?highlight=string#return-spans-when-possible