Phase 4: CHAIN, RUN "file", random-access I/O, MBF conversions

Add CHAIN statement for loading and running chained programs with
optional ALL flag to preserve variables. Extend RUN to accept a
filename string argument. Implement random-access file I/O with
FIELD, LSET, RSET, PUT#, GET# and the MBF conversion functions
CVI/CVS/CVD/MKI$/MKS$/MKD$. Add COMMON statement (parse and skip).
Five new test programs covering all new features (27 total).

tests/programs/mkicvi.bas

@@ -0,0 +1,7 @@
+10 REM MKI$/CVI, MKS$/CVS, MKD$/CVD test
+20 A$ = MKI$(12345)
+30 PRINT CVI(A$)
+40 B$ = MKS$(3.14)
+50 PRINT CVS(B$)
+60 C$ = MKD$(2.718281828)
+70 PRINT CVD(C$)