[framework] Adds bcs::peel_enum_tag

[damirka]

Description

Minor improvement for bcs library, helper to parse enums.

Test plan

Features tests.

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Release notes

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• Protocol: Changes to the Sui framework, introduction of bcs::peel_enum_tag function
• Nodes (Validators and Full nodes):
• gRPC:
• JSON-RPC:
• GraphQL:
• CLI:
• Rust SDK:

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[tnowacki]

I am thinking that this shouldn't really be a macro after some further reading.
peel_vec and peel_option both have a return type of their constructing type--vector and Option respectively.

But peel_enum here is just passing back the return value of peel_enum_tag.

For example

public macro fun peel_enum<$T>($bcs: &mut BCS, $f: |u64| -> $T): $T
...
bcs.peel_enum!(|tag| match (tag) {
    0 => Enum::Empty,
    1 => Enum::U8(bcs.peel_u8()),
    2 => Enum::U16(bcs.peel_u16()),
    _ => abort,
}

could just be

public fun peel_enum_tag(bcs: &mut BCS): u64
...
match (peel_enum_tag(tag)) {
    0 => Enum::Empty,
    1 => Enum::U8(bcs.peel_u8()),
    2 => Enum::U16(bcs.peel_u16()),
    _ => abort,
}

TLDR the macro is just function application, which isn't really that interesting I think?