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/data_read.bas

@@ -0,0 +1,9 @@
+10 REM DATA/READ test
+20 DATA 10, 20, 30, "hello", "world"
+30 READ A, B, C
+40 PRINT A; B; C
+50 READ D$, E$
+60 PRINT D$; " "; E$
+70 RESTORE
+80 READ X
+90 PRINT "After RESTORE:"; X