Begin adding parser tests; fix unary +/- inconsistency

Don't have a leading +/- be part of the regex for literal numbers as 
this subtly changes the logic for numbers vs other expressions: `-5**2` 
should evaluate to -25 because it is *meant* to parse as `-(5**2)`. 
However, with the current logic, the `-` is parsed as part of the number 
and so this parses as `(-5)**2`, which evaluates to 25.

src/flitter/language/grammar.lark

@@ -10,7 +10,7 @@ NAME : /[_\p{alpha}][_\p{alnum}]*'*/i
 NODE : /![_\p{alpha}][_\p{alnum}]*'*/i
 TAG : /#[_\p{alpha}][_\p{alnum}]*'*/i
 SYMBOL : /:[_\p{alpha}][_\p{alnum}]*'*/i
-NUMBER : /[-+]?([0-9][_0-9]*(\.[0-9][_0-9]*)?|\.[0-9][_0-9]*)([eE][-+]?[0-9][_0-9]*)?[pnuµmkMGT]?/
+NUMBER : /([0-9][_0-9]*(\.[0-9][_0-9]*)?|\.[0-9][_0-9]*)([eE][-+]?[0-9][_0-9]*)?[pnuµmkMGT]?/
 STRING : /('''([^'\\]|\\.|'{1,2}(?!'))*'''|'([^'\n\\]|\\.)*'|"""([^"\\]|\\.|"{1,2}(?!"))*"""|"([^"\n\\]|\\.)*")/
 _HASHBANG : /#!.*\r?\n/
 _LPAREN : "("

tests/test_parser.py

@@ -0,0 +1,43 @@
+"""
+Flitter parser tests
+"""
+
+import unittest
+
+from flitter.language.tree import (
+    Literal, Name, Top, Export, Sequence,
+    Positive, Negative, Power, Add, Subtract, Multiply, Divide, FloorDivide, Modulo,
+    # Ceil, Floor, Fract,
+    # Contains, EqualTo, NotEqualTo, LessThan, GreaterThan, LessThanOrEqualTo, GreaterThanOrEqualTo,
+    # Not, And, Or, Xor,
+    # Range, Slice, Lookup,
+    # Tag, Attributes, Append,
+    # Let, Call, For, IfElse,
+    # Import, Function,
+    # Binding, PolyBinding, IfCondition
+)
+from flitter.language.parser import parse
+
+
+class ParserTestCase(unittest.TestCase):
+    def assertParsesTo(self, code, expression):
+        self.maxDiff = 1000
+        if not isinstance(expression, Top):
+            if not isinstance(expression, Sequence):
+                expression = Sequence((expression, Export(None)))
+            elif not isinstance(expression.expressions[-1], Export):
+                expression = Sequence(expression.expressions + (Export(None),))
+            expression = Top((), expression)
+        self.assertEqual(repr(parse(code)), repr(expression))
+
+
+class TestPrecedence(ParserTestCase):
+    def test_maths_literals(self):
+        self.assertParsesTo("0-1//2%3++4*-5**6/7",
+                            Add(Subtract(Literal(0), Modulo(FloorDivide(Literal(1), Literal(2)), Literal(3))),
+                                Divide(Multiply(Positive(Literal(4)), Negative(Power(Literal(5), Literal(6)))), Literal(7))))
+
+    def test_maths_names(self):
+        self.assertParsesTo("a-b//c%d++e*-f**g/h",
+                            Add(Subtract(Name('a'), Modulo(FloorDivide(Name('b'), Name('c')), Name('d'))),
+                                Divide(Multiply(Positive(Name('e')), Negative(Power(Name('f'), Name('g')))), Name('h'))))