Added clear method on HashMapContext's variables and functions fields. (

#156)

I've the below use case where numerous allocations are happening and
hoping to reduce the impact of those re-allocations using the _clear_
method on the fields of `HashMapContext`:

There are 1000s of transactions with each transaction having about 40
fields. Let's say field 5 and field 30 are sent from source as optional
Boolean type (`Option<bool>`), which would mean certain transaction
could have a boolean value and other transactions would have null value.
I cannot use a default boolean value here. Since once the
`HashMapContext` variables are created, their types can no longer be
changed. Therefore, I have re-initiate the HashMapContext ```let mut
context = HashMapContext::new();``` after every transaction and does the
re-allocation.

src/context/mod.rs

@@ -185,6 +185,16 @@ impl HashMapContext {
     pub fn new() -> Self {
         Default::default()
     }
+
+    /// Clears the [variables] HashMap so it can re-populated if needed
+    pub fn clear_variables_map(&mut self) {
+        self.variables.clear()
+    }
+
+    /// Clears the [functions] HashMap so it can re-populated if needed
+    pub fn clear_functions_map(&mut self) {
+        self.functions.clear()
+    }
 }
 
 impl Context for HashMapContext {