Parsing negative numbers a-la ruby, closes #383 and #382

This bugfix introduces negative numbers into our parser:
before, -10 would be parsed as (minus, number) whereas
right now it's going to be parsed as (number) only.

This fixes a bug with precedences, where `-1.str()`
would be parsed as (-, (1, str)), leading to first
calling the str method on the positive number, then
applying the minus.

Before:

```
⧐  -1.234.str()
ERROR: unknown operator: -STRING
	[1:1]	-1.234.str()

```

After:

```
⧐  -1.234.str()
-1.234
```

If a space is found between the minus and the number,
the old parsing mechanism still applies.

I found inspiration in Ruby, where:

```
$ - 1.to_s()
-@: undefined method `-@' for "1":String

$-1.to_s()
-1

$ -10.3.floor()
-11

$ - 10.3.floor()
-10
```

VERSION

@@ -1 +1 @@
-2.2.1
+2.2.2

evaluator/builtin_functions_test.go

@@ -553,6 +553,8 @@ func TestBetween(t *testing.T) {
 		{`1.between(0, 0.9)`, false},
 		{`1.between(1, 0)`, "arguments to between(min, max) must satisfy min < max (1 < 0 given)"},
 		{`1.between(1, 2)`, true},
+		{`-1.between(-10, 0)`, true},
+		{`-1.between(-10, -2)`, false},
 	}
 
 	testBuiltinFunction(tests, t)

lexer/lexer.go

@@ -107,6 +107,13 @@ func (l *Lexer) NextToken() token.Token {
 			tok.Position = l.position
 			tok.Literal = "-="
 			l.readChar()
+		} else if isDigit(l.peekChar()) {
+			tok.Position = l.position
+			l.readChar()
+			literal, kind := l.readNumber()
+			tok.Type = kind
+			tok.Literal = "-" + literal
+			return tok
 		} else {
 			tok = l.newToken(token.MINUS)
 		}

lexer/lexer_test.go

@@ -9,6 +9,9 @@ import (
 func TestNextToken(t *testing.T) {
 	input := `five = 5;
 ten = 10;
+-10;
+- 10;
+1 - 2;
 add = f(x, y) {
   x + y;
 };
@@ -127,6 +130,15 @@ f hello(x, y) {
 		{token.ASSIGN, "="},
 		{token.NUMBER, "10"},
 		{token.SEMICOLON, ";"},
+		{token.NUMBER, "-10"},
+		{token.SEMICOLON, ";"},
+		{token.MINUS, "-"},
+		{token.NUMBER, "10"},
+		{token.SEMICOLON, ";"},
+		{token.NUMBER, "1"},
+		{token.MINUS, "-"},
+		{token.NUMBER, "2"},
+		{token.SEMICOLON, ";"},
 		{token.IDENT, "add"},
 		{token.ASSIGN, "="},
 		{token.FUNCTION, "f"},

parser/parser.go

@@ -469,7 +469,6 @@ func (p *Parser) parsePrefixExpression() ast.Expression {
 	}
 
 	p.nextToken()
-
 	expression.Right = p.parseExpression(PREFIX)
 
 	return expression

parser/parser_test.go

@@ -171,7 +171,7 @@ func TestParsingPrefixExpressions(t *testing.T) {
 		value    interface{}
 	}{
 		{"!5;", "!", 5},
-		{"-15;", "-", 15},
+		{"- 15;", "-", 15},
 		{"!foobar;", "!", "foobar"},
 		{"-foobar;", "-", "foobar"},
 		{"!true;", "!", true},
@@ -310,6 +310,10 @@ func TestOperatorPrecedenceParsing(t *testing.T) {
 		},
 		{
 			"3 + 4; -5 * 5",
+			"(3 + 4)(-5 * 5)",
+		},
+		{
+			"3 + 4; - 5 * 5",
 			"(3 + 4)((-5) * 5)",
 		},
 		{