Introduce wrapper type for asset amounts

[PhilippGackstatter]

We should add a dedicated type that represents a valid amount in a FungibleAsset. This could be called FungibleAssetAmount or AssetAmount. The former is a bit clearer to which type it belongs while the latter is more concise. I think I'd go with the former.

It should probably have at least these parts:

#[derive(Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
pub struct FungibleAssetAmount(u64);

impl FungibleAssetAmount {
    // Replaces from FungibleAsset::MAX_AMOUNT
    pub const MAX: Self = Self(2u64.pow(63) - 2u64.pow(31));

    /// Returns an error if amount exceeds Self::MAX.
    pub fn new(amount: u64) -> Result<Self, _> { ... }
}

impl From<u8> for FungibleAssetAmount { ... }
impl From<u16> for FungibleAssetAmount { ... }
impl From<u32> for FungibleAssetAmount { ... }
impl TryFrom<u64> for FungibleAssetAmount { ... }

impl core::fmt::Display for FungibleAssetAmount { ... }

We should definitely avoid constructors that take an arbitrary u64 and round it down to % FungibleAssetAmount::MAX.

The need for this came up in a few places:

• Refactor MINT note to support public output notes #2073 (comment)
• feat: adapt the layout of Assets to the double word representation #2437 (comment)

[bobbinth]

Not a strong preference, but I do like AssetAmount a bit better mostly because it is concise, and we can't have any other asset amounts (e.g., AssetAmount kind of implies that this is for a fungible token).

[giwaov]

I'd like to take on this issue. I'll introduce the AssetAmount wrapper type as discussed (using bobbinth's preferred concise name), with validated construction, From<u8/u16/u32>, TryFrom<u64>, and Display. Will integrate it into FungibleAsset and update all downstream usages.