Use Slice.literal for fast_float when supported, part 2

[HertzDevil]

Follow-up to #15667.

[straight-shoota]

thought: The value lists are identical between both types. We could reduce code duplication by making only the outer layer conditional:

{% begin %}
  {% if compare_versions(Crystal::VERSION, "1.16.0") < 0 %}
    [
  {% else %}
    Slice(UInt64).literal(
  {% end %}
  1_u64,
  # ...
  7450580596923828125_u64,
  {% if compare_versions(Crystal::VERSION, "1.16.0") < 0 %}
    ]
  {% else %}
    )
  {% end %}
{% end %}

The added overhead won't make this much appealing for very short slices. But it might be a good idea for the bigger ones? WDYT?

[straight-shoota]

ping @HertzDevil any thoughts on this?
It shouldn't be a blocker, so I'd like to move on if you don't think it's worth it.

[ysbaddaden]

Duplicating the values is unfortunate but the alternative impacts readability, so I'm not sure it's really better.

Aside: maybe a macro def could help if we supported them, though it would be very inefficient (lots of allocations & transformations) so I'm not sure it would be very useful, especially for large literals and static data that won't change.

macro def slice_literal(type, *values)
  {% if compare_versions(Crystal::VERSION, "1.16.0") < 0 %}
    Slice({{type}}.literal({{values.splat}})
  {% else %}
    [{{values.splat}}]
  {% end %}
end

LARGE_POWER_OF_5 = {{ slice_literal(UInt64,
  1414648277510068013_u64,
  9180637584431281687_u64,
  4539964771860779200_u64,
  10482974169319127550_u64,
  198276706040285095_u64,
) }}