seq: use BigDecimal to represent floats (#2698)

* 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.

* fixup! seq: use BigDecimal to represent floats

* fixup! seq: use BigDecimal to represent floats

* fixup! seq: use BigDecimal to represent floats

* fixup! seq: use BigDecimal to represent floats

* fixup! seq: use BigDecimal to represent floats

Author: jfinkels

Cargo.lock

@@ -0,0 +1,19 @@
+# Benchmarking to measure performance
+
+To compare the performance of the `uutils` version of `seq` with the
+GNU version of `seq`, you can use a benchmarking tool like
+[hyperfine][0]. On Ubuntu 18.04 or later, you can install `hyperfine` by
+running
+
+    sudo apt-get install hyperfine
+
+Next, build the `seq` binary under the release profile:
+
+    cargo build --release -p uu_seq
+
+Finally, you can compare the performance of the two versions of `head`
+by running, for example,
+
+    hyperfine "seq 1000000" "target/release/seq 1000000"
+
+[0]: https://github.com/sharkdp/hyperfine

src/uu/seq/BENCHMARKING.md

@@ -1,3 +1,4 @@
+# spell-checker:ignore bigdecimal
 [package]
 name = "uu_seq"
 version = "0.0.8"
@@ -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"