Fix code to match latest lrgrep version

[let-def]

Hi,

I am preparing for a first alpha release of LRgrep. The breaking changes are not happening in master for the moment but in three separate branches:

• enumerate which adds a tool for fuzzing a parser
• coverage which adds a tool for checking exhaustive coverage of all syntax errors
• parser-tweak where I cleanup the user experience, also introducing some breaking changes in the syntax and the API.

I will keep this branch of illuaminate synchronized to build with parser-tweak. The two changes relevant to illuaminate (for now):

1. partial keyword is now written %partial. This is to avoid an ambiguity with a grammar already having a partial symbol. (I tested LRgrep on a grammar which had a rule non-terminal; luckily the grammar could be updated to avoid the confusion, only partial remained problematic and I want to make sure that all Menhir symbols can be recognized unambiguously).
2. the generated code has a friendlier (I think) API which no longer require to instantiate a module and to check the candidates manually.