Show line number in runtime panic

[jclark]

Preferably column too.

This requires passing line/column info through the compilation pipeline from parsing to LLVM building.

[jclark]

Plan:

• for AST node for binary + - * / %, and unary -, include position
• include that also in relevant BIR insn
• when building LLVM, put the column and line in the argument passed to panic (we can either make a LLVM struct or do the encoding with bitwise operations ourself)

[jclark]

This will require changing AST, so this one is waiting on #50. Apart from adding position info, I want to separate out the arithmetic binary ops from the other ones.

[jclark]

When we do this, we also need to fix testll.sh to check that the line number is correct.

[jclark]

Handle type casts too: they already have the position in BIR.

[jclark]

• Separate out arithmetic binary ops from other ones
• Include position in AST for arithmetic binary ops
• Include position in BIR for arithmetic binary ops
• Pass position information to panic function for arithmetic ops
• Pass position information to panic function for type cast
• Fix runtime to print out position information if present
• Fix testll.sh to check that line number is correct