Integer and Decimal Type Renames for 2.0

[younies]

Discussion: core decimal types

• @sffc Did we want pub type UnsignedDecimal = WithDecimal<UnsignedInteger>? Decimal is an Integer with an extra field that specifies the location of the decimal point.
• @robertbastian Yes, but it is extra work.
• @sffc Is it breaking to change pub struct UnsignedDecimal to pub type UnsignedDecimal?
• @robertbastian Maybe, not sure
• @sffc If we keep fixed_decimal as a separate non-stable crate, this is fine, because the only thing we format is Decimal
• @sffc Do we want the type wrap to be private? pub struct Decimal(Signed<UnsignedDecimal>)
• @robertbastian That requires duplicating all the functions or derefing, and removing a deref step seems more semver breaking than the alternative

Proposal:

1. Definitely in 2.0:
   • pub type Decimal = Signed<UnsignedDecimal>
   • pub struct UnsignedDecimal
2. Additional integer renames (see point 3):
   • pub type Integer = Signed<UnsignedInteger>
   • pub struct UnsignedInteger
3. If fixed_decimal remains a util crate, we can do the integer changes post-2.0, which may involve a range dependency from icu_decimal to fixed_decimal. If we merge it into icu_decimal, we put the integer types behing an experimental feature.
4. If we can make pub type UnsignedDecimal in 2.x, great, if not, fine, maybe in 3.0

LGTM: @sffc @younies @robertbastian @echeran