Add calculator core: constants, operations and tests

MIL-002 Calculator core.

- Add constants.py with operation symbols, answers, prompts, messages
  and the ASCII title and calculator
- Add add, subtract, multiply and divide, and the OPERATIONS dictionary
  that maps each symbol to its function without calling it
- Add tests for each operation, division by zero and the dictionary

Closes #19
Closes #20
Closes #21

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3

tests/test_operations.py

@@ -0,0 +1,33 @@
+"""!
+@file test_operations.py
+@brief Tests for the arithmetic operations.
+"""
+
+import pytest
+
+from calc.operations import OPERATIONS, add, divide, multiply, subtract
+
+
+def test_add_returns_sum() -> None:
+    assert add(2, 3) == 5
+
+
+def test_subtract_returns_difference() -> None:
+    assert subtract(2, 3) == -1
+
+
+def test_multiply_returns_product() -> None:
+    assert multiply(4, 2.5) == 10
+
+
+def test_divide_returns_quotient() -> None:
+    assert divide(9, 3) == 3
+
+
+def test_divide_when_divisor_is_zero() -> None:
+    with pytest.raises(ZeroDivisionError):
+        divide(1, 0)
+
+
+def test_operations_maps_symbols_to_functions() -> None:
+    assert OPERATIONS == {"+": add, "-": subtract, "*": multiply, "/": divide}