Programs should store DEFCIRCUITs in a map like other "DEF" instructions

[MarquessV]

Currently, DEFCIRCUIT is classified as part of a Program's "body instructions" whereas all other "DEF" instructions are stored in their own mapping.

Since body instructions are stored in a Vec, it's not possible to find a DEFCIRCUIT in a program without doing a O(n) search through the body_instructions vector. DEFCIRCUITs are often re-used in multiple programs, and storing them in a map would allow them to be accessed more efficiently.

From a UX perspective, it is also confusing that all DEF instructions except for DEFCIRCUIT get hoisted to the top of the program.

[MarquessV]

cc: @mhodson-rigetti @bramathon