Gensym improvements

[ceastlund]

Improved two aspects of gen_symbol:

1. passing one gen_symbol result as ~prefix to another no longer results in a double-suffix
2. name_type_params_in_td produces more sensible variable names
   First added tests for both behaviors, then improved them and updated tests.

[pitag-ha]

Thanks, LGTM!

I'm not very familiar with name_type_params_in_td. Why is it better to use a, b, ... as a prefix for the name instead of v_x or no prefix at all?

[ceastlund]

The name_type_params_in_td function assigns names to type parameters declared as underscores, because having names for them is often handy during ppx expansion. Let's say we start with:
type (_, _) t = int
Currently, name_type_params_in_td produces something like:
type ('v_x__097_, 'v_x__098_) t = int
The code is written using "v" ^ gen_symbol (), and gen_symbol has a ?prefix argument that defaults to "_x". I kind of suspect the code meant to use "v" as a prefix, and the author didn't think about the "_x" default. I suspect the intention was:
type ('v__097_, 'v__098_) t = int
There's no real reason why "_x" or "v_x" are at all meaningful or suggestive names for types. So I wanted to change that code to use gen_symbol's ~prefix argument.

While I was at it, it occurred to me that "v" is also not a great prefix for a type. Usually, v is for "value". We usually use the beginning of the alphabet, like:
type ('a, 'b) t = int
So I made the code do something similar:
type ('a__097_, 'b__098_) t = int

[pitag-ha]

Oh nice, thanks for all the explanation! Using a..., b..., ... for type parameter names does indeed sounds more intuitive than v_... or v_x.... As a side-note: it's a..., b..., ..., not 'a..., 'b..., ..., right?

[ceastlund]

What gen_symbol generates has no apostrophe. But when wrapped in Ptyp_var and rendered back as ocaml syntax, there would be an apostrophe there.