Phase 2: variables, arrays, program execution, and control flow

Add variable storage (scalar and array), program line storage with
RUN/LIST/NEW, and full control flow: FOR/NEXT, GOTO, GOSUB/RETURN,
IF/THEN/ELSE, WHILE/WEND, ON GOTO/GOSUB, DATA/READ/RESTORE.

New modules: interp.c (1445 lines - execution loop and statement
dispatcher), vars.c, arrays.c, input.c. Version bumped to 0.2.0.
16 test programs pass including Leibniz pi and prime sieve.

tests/programs/while_wend.bas

@@ -0,0 +1,8 @@
+10 REM WHILE/WEND test
+20 X = 1
+30 WHILE X <= 10
+40 IF X MOD 2 = 0 THEN PRINT X;
+50 X = X + 1
+60 WEND
+70 PRINT
+80 PRINT "Done"