units: Make amount module not require "alloc" feature

[tcharding]

The amount module has loads of string parsing and heavily relies on types from allloc, however there may be no-std users that would still like to have an Amount type. It should be feasible to make the feature gating more fine grained so we can provide an Amount type in "no-std" builds.

[Kixunil]

I thought this is done already? Anyway, can look into it after #2370 merges.

[tcharding]

The whole amount module is feature gated on "alloc"

// TODO: Make amount module less dependent on an allocator.
#[cfg(feature = "alloc")]
pub mod amount;