Remove Array8, Array16 and Array32 types

[Kmeakin]

• Removes the Array8, Array16 and Array32 types and their corresponding functions, leaving only Array64.
• Added <from>_extend_<to> and <from>_truncate_<to> for all integer types
• Implicitly insert extension functions during coercion. This allows using shorter integers (eg parsed from input) to operate on Array64 without hassle

[Kmeakin]

Removed implicit extension because it could silently change the result of expressions (eg (u16_extend_u64 x * u16_extend_u64 y) != u16_extend_u64 (x * y))