Add support for arbitrary-precision floats

Using [rug](https://docs.rs/rug/1.11.0/rug/index.html) Rationals.

Closes #261

libslide/Cargo.toml

@@ -25,7 +25,7 @@ required-features = ["benchmark-internals"]
 bitflags = "1.2.1"
 lasso = "0.3.1"
 lazy_static = "1.4.0"
-strtod = "0.0.1"
+rug = "1.11.0"
 
 [dependencies.num-traits]
 default-features = false

libslide/src/common.rs

@@ -87,3 +87,33 @@ impl From<Span> for (usize, usize) {
         (span.lo, span.hi)
     }
 }
+
+/// Context to use across a slide program.
+#[derive(Copy, Clone)]
+pub struct ProgramContext {
+    /// Precision to use for [Float][rug::Float]s.
+    pub(crate) prec: u32,
+}
+
+/// Dummy FP precision, only for use in tests or where precision is not relevant.
+static DUMMY_PREC: u32 = 200;
+
+impl Default for ProgramContext {
+    /// Only to be used in situations where the program context is irrelevant; i.e. expressions are
+    /// being used outside of the primary program.
+    fn default() -> Self {
+        Self { prec: DUMMY_PREC }
+    }
+}
+
+impl ProgramContext {
+    /// Creates a new `ProgramContext`.
+    pub fn new(prec: u32) -> Self {
+        Self { prec }
+    }
+
+    #[cfg(test)]
+    pub(crate) fn test() -> Self {
+        Self::default()
+    }
+}