banning `!` suffix from keywords

Stream: lean4

[Leonardo de Moura]

@Andrew Kent and @Joe Hendrix pointed out that we were using ! inconsistently. It was used in functions that may invoke "panic", macros and builtin notation . So, we decided to remove ! suffix from all macros, but panic!, assert!, and unreachable! since they are just syntax sugar on top of panic. We are also enforcing the use of snake_case for keywords. For example, natLit! is now called nat_lit.