Implemented lexer

tests/week_1/valid/multi_digit.c

@@ -0,0 +1,3 @@
+int main() {
+    return 100;
+}

tests/week_1/valid/newlines.c

@@ -0,0 +1,10 @@
+
+int 
+main
+(   
+)
+{
+return
+0
+;
+}

tests/week_1/valid/no_newlines.c

@@ -0,0 +1 @@
+int main(){return 0;}

tests/week_1/valid/return_0.c

@@ -0,0 +1,3 @@
+int main() {
+    return 0;
+}

tests/week_1/valid/return_2.c

@@ -0,0 +1,3 @@
+int main() {
+    return 2;
+}

tests/week_1/valid/spaces.c

@@ -0,0 +1 @@
+   int   main    (  )  {   return  0 ; }