seq: use BigDecimal to represent floats

Use `BigDecimal` to represent arbitrary precision floats in order to
prevent numerical precision issues when iterating over a sequence of
numbers. This commit makes several changes at once to accomplish this
goal.

First, it creates a new struct, `PreciseNumber`, that is responsible for
storing not only the number itself but also the number of digits (both
integer and decimal) needed to display it. This information is collected
at the time of parsing the number, which lives in the new
`numberparse.rs` module.

Second, it uses the `BigDecimal` struct to store arbitrary precision
floating point numbers instead of the previous `f64` primitive
type. This protects against issues of numerical precision when
repeatedly accumulating a very small increment.

Third, since neither the `BigDecimal` nor `BigInt` types have a
representation of infinity, minus infinity, minus zero, or NaN, we add
the `ExtendedBigDecimal` and `ExtendedBigInt` enumerations which extend
the basic types with these concepts.

Author: jfinkels

Cargo.lock

@@ -1,3 +1,4 @@
+# spell-checker:ignore bigdecimal
 [package]
 name = "uu_seq"
 version = "0.0.7"
@@ -15,6 +16,7 @@ edition = "2018"
 path = "src/seq.rs"
 
 [dependencies]
+bigdecimal = "0.3"
 clap = { version = "2.33", features = ["wrap_help"] }
 num-bigint = "0.4.0"
 num-traits = "0.2.14"