' not allowed at the beginning of an identifier

[fblanqui]

https://github.com/Deducteam/Dedukti/blob/3dbbffdb5c8af3be15bb2c7a59ba263505c3a360/parsing/lexer.mll#L25

it may be useful to write 'a like in ML and systems based on ML (Isabelle, HOL-Light, HOL4)