bcal v2.6

bcal v2.6 release notes:

• support native prompt and history without readline (make O_NORL=1)
• add strip and static build (make O_STATIC=1) options
• show prompt help and allow c and p commands in either REPL mode
• add BCAL_BIT_ANSI_COLOR_CODE to customize set bit color
• add sum() function for whitespace-separated numbers
• add -H option to show integral maths results in hexadecimal
• recognize space-separated b or B as byte unit