Custom Gambit-c R7RS bindings for raylib

For now has the following files:

• raylib.sld
  • R7RS library definition
• structs.scm
  • raylib specific structs
• drawing.scm
  • raylib drawing-related functions
• timing.scm
  • raylib timing-related functions
• input.scm
  • raylib input-related functions for keyboard & mouse
• shapes.scm
  • rshapes module

Additionally defines the two hygienic macros

• with-drawing
  • opens a raylib drawing context, evals body, then closes the context
• in-scissor-mode
  • opens scissor-mode, evals body, then closes scissor-mode