[Internal] change `Pconst_char` char to `Pconst_char int`

[bobzhang]

This is a pre-requisite of #5749 to avoid Obj.magic everywhere
char is essentially an int, the impact is expected to be minimal
To make this happen, we need make changes in two repos: syntax and compiler

cc @butterunderflow @IwanKaramazow

[cristianoc]

That should be OK.