Refactor closure_tyvars_let_decl to avoid type variable duplication

[oshimayuki1124]

When evaluating the following expression:

let g = fun x -> ((fun y -> y): ? -> ?) x

type abstractions are inserted as:

let g = fun 'x10, 'x11, 'x10 -> fun (x: 'x11) -> ((fun (y: 'x10) -> y): 'x10 -> 'x10 => ? -> ?) (x: 'x11 => ?)

As seen here, the type variable 'x10 appears twice. This duplication happens because the return values of closure_tyvars_let_decl and closure_tyvars2 are not mutually exclusive.

To address this, I made the following changes:

• Introduced a new function closure_tyvars_let_decl2, which behaves like closure_tyvars2 but excludes tv_exp v1 instead of ftv_exp v1.
• Renamed closure_tyvars_let_decl to closure_tyvars_let_decl1.

This change does not affect practical behavior, but it may be relevant in theoretical contexts.

Let me know if you have any feedback or if this can be merged.