Make AssetId::from_inner a const function

[pavel-kokolemin]

This will allow constructing AssetId values like this:

const TESTNET_ASSET: elemets::AssetId = const_asset_id("144c654344aa716d6f3abcc1ca90e5641e4e2a7f633bc09fe3baf64585819a49");

/// Panics if the asset id string has wrong length or format
pub const fn const_asset_id(s: &str) -> elements::AssetId {
    let mut data: [u8; 32] = hex_literal::decode::<32>(&[s.as_bytes()]);

    let mut left = 0;
    let mut right = data.len() - 1;
    while left < right {
        let tmp = data[left];
        data[left] = data[right];
        data[right] = tmp;
        left += 1;
        right -= 1;
    }

    elements::AssetId::from_inner(bitcoin::hashes::sha256::Midstate(data))
}

It's a bit of a hack because hex_literal::decode should not be used directly.

[apoelstra]

ACK 1f3b402; successfully ran local tests; nice, good call