R script tests

[Jakobeha]

Create the test harness which:

• Reads an R script consisting of a sequence of closure definitions
• Generates cached/snapshot .rds files with the AST and bytecode of each closure (2 files per closure)
• Compares the result of reading the AST closure and compiling it on our side, with reading the bytecode closure GNU-R compiled, and asserts they are structurally equivalent
• Also does a snapshot test on the .toString() of our bytecode representation, so we don't regress deserialization in a way that keeps the bytecodes structurally equivalent