Semantic analysis

[ilammy]

Initial implementation of semantic analysis

• [x] Basic meaning module, test harness
• [x] Handling expressions
  • [x] Literals
  • [x] Quotes
  • [x] References
  • [x] Conditions
  • [x] Assignments
  • [x] Sequences
  • [x] Abstractions
  • [x] Applications
• [x] Reporting diagnostics
  • [x] Undefined references
  • [x] Assignment to undefined variables
  • [x] Assignment to immutable variables
• [x] Batch processing of expressions
• [x] begin splicing
• [x] Gather constants
• [x] Improve pretty-printing (use Debug trait instead)
• [x] Builder for test cases