Optional array check mode

[richfitz]

To help with debugging it would be useful to compile in a "safe" mode where all array access is bounds checked. That would involve replacing all

x[i]

with an accessor function. This will have potentially very large performance costs so we'd want to be able to turn this on and off easily but probably at compile time

[richfitz]

See #49

[richfitz]

The use of size_t for i really exposes us to catastrophic underflows if we have i - 1 on array indexing...

[richfitz]

Pending:

• underflow
• multidimensional array
• error if used on non-array code