Add no_std support and testing

Cargo.toml

@@ -11,6 +11,11 @@ edition = "2018"
 [dev-dependencies]
 criterion = "0.3"
 
+[features]
+default = ["std"]
+alloc = []
+std = ["alloc"]
+
 [[bench]]
 name = "stock_functions"
 harness = false

Jenkinsfile

@@ -16,6 +16,16 @@ pipeline {
              sh "cargo test"
          }
      }
+     stage('Check no_std') {
+         steps {
+             sh "cargo check --no-default-features --features alloc"
+         }
+     }
+     stage('Test no_std') {
+         steps {
+             sh "cargo test --no-default-features --features alloc"
+         }
+     }
      stage('Coverage') {
          steps {
              sh "cargo clean"

src/bigint.rs

@@ -2,6 +2,17 @@
 //! \[WIP\] Arbitrarily large integers
 use crate::num::Sign::*;
 use crate::num::*;
+
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+extern crate alloc;
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+use alloc::vec::*;
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+use alloc::string::*;
+
+#[cfg(feature = "std")]
+use std::prelude::v1::*;
+
 use core::convert::TryInto;
 use core::mem::replace;
 use core::ops::{

src/lib.rs

@@ -2,8 +2,18 @@
 //!
 //! Playin' with Numerics in Rust
 #![forbid(unsafe_code)]
+#![no_std]
 
-use std::f64::consts::{E, PI};
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+#[macro_use]
+extern crate alloc;
+
+#[cfg(feature = "std")]
+#[macro_use]
+extern crate std;
+
+#[cfg(feature = "std")]
+use core::f64::consts::{E, PI};
 
 pub mod bigint;
 pub mod num;
@@ -183,6 +193,7 @@ pub fn factorial(n: usize) -> Option<u128> {
 /// let invalid = approx_factorial(171.0); // None
 /// # assert!(invalid.is_none());
 /// ```
+#[cfg(feature = "std")]
 #[inline]
 pub fn approx_factorial(n: f64) -> Option<f64> {
     let power = (n / E).powf(n);
@@ -233,6 +244,7 @@ mod tests {
         assert!(it_factorial(35).is_none());
     }
 
+    #[cfg(feature = "std")]
     #[test]
     fn test_approx_factorial() {
         assert!(approx_factorial(170.624).is_some());

src/rational.rs

@@ -2,8 +2,8 @@
 
 use crate::num::Sign::*;
 use crate::num::*;
-use std::cmp::{Ord, Ordering, PartialOrd};
+use core::cmp::{Ord, Ordering, PartialOrd};
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Sub, SubAssign};
+use core::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
 /// Type representing a fraction
 ///