Fix VisibleString character set to follow X.680 ISO/IEC 646 definition

[mp0rta]

fix #191

This PR fixes an incorrect conflation of VisibleString and PrintableString during
character-set validation.

VisibleString was previously validated against the PrintableString character
repertoire, which contradicts the VisibleString definition in X.680 and causes
valid VisibleString characters to be rejected when evaluating permitted-alphabet
constraints.

Changes

• Separate VisibleString character-set handling from PrintableString
• Model VisibleString using the ISO/IEC 646 visible character range (0x20..=0x7E)
• Preserve existing PrintableString behavior

Result

• Valid VisibleString characters such as '_' and '~' are no longer rejected
• Permitted-alphabet constraints behave as specified in X.680
• PrintableString semantics remain unchanged

[mp0rta]

I post the test result.
and I've checked the cargo test pass.

test asn file

TestVisibleStringConstraint DEFINITIONS ::= BEGIN

T ::= VisibleString
        (FROM ("a".."z" | "A".."Z" | "0".."9" | ":./-_@"))
        (SIZE (1..255))

END

before revision

Warnings:
Unidentified generating bindings for : Character _ is not in char set: {0: 'A', 1: 'B', 2: 'C', 3: 'D', 4: 'E', 5: 'F', 6: 'G', 7: 'H', 8: 'I', 9: 'J', 10: 'K', 11: 'L', 12: 'M', 13: 'N', 14: 'O', 15: 'P', 16: 'Q', 17: 'R', 18: 'S', 19: 'T', 20: 'U', 21: 'V', 22: 'W', 23: 'X', 24: 'Y', 25: 'Z', 26: 'a', 27: 'b', 28: 'c', 29: 'd', 30: 'e', 31: 'f', 32: 'g', 33: 'h', 34: 'i', 35: 'j', 36: 'k', 37: 'l', 38: 'm', 39: 'n', 40: 'o', 41: 'p', 42: 'q', 43: 'r', 44: 's', 45: 't', 46: 'u', 47: 'v', 48: 'w', 49: 'x', 50: 'y', 51: 'z', 52: '0', 53: '1', 54: '2', 55: '3', 56: '4', 57: '5', 58: '6', 59: '7', 60: '8', 61: '9', 62: ' ', 63: '\'', 64: '(', 65: ')', 66: '+', 67: ',', 68: '-', 69: '.', 70: '/', 71: ':', 72: '=', 73: '?'}


Generated:
#[allow(
    non_camel_case_types,
    non_snake_case,
    non_upper_case_globals,
    unused,
    clippy::too_many_arguments
)]
pub mod test_visible_string_constraint {
    extern crate alloc;
    use core::borrow::Borrow;
    use rasn::prelude::*;
    use std::sync::LazyLock;
}

after revision

Generated:
#[allow(
    non_camel_case_types,
    non_snake_case,
    non_upper_case_globals,
    unused,
    clippy::too_many_arguments
)]
pub mod test_visible_string_constraint {
    extern crate alloc;
    use core::borrow::Borrow;
    use rasn::prelude::*;
    use std::sync::LazyLock;
    #[doc = " Minimal reproduction for permitted alphabet + size constraints on VisibleString"]
    #[derive(AsnType, Debug, Clone, Decode, Encode, PartialEq, Eq, Hash)]
    #[rasn(
        delegate,
        size("1..=255"),
        from(
            "\u{2d}",
            "\u{2e}",
            "\u{2f}",
            "\u{30}..=\u{39}",
            "\u{3a}",
            "\u{40}",
            "\u{41}..=\u{5a}",
            "\u{5f}",
            "\u{61}..=\u{7a}"
        )
    )]
    pub struct T(pub VisibleString);
}

[6d7a]

Thank you for your contribution!