Compatibility with 32-bit platforms

[UlfNorell]

(Context: #29)

Agda doesn't have any builtin 32-bit numerical types, so it's not clear how to best support 32-bit platforms (if at all). Some options (in order of less work to more work):

• Map Agda 64-bit numbers to Int and Word, but issue a warning on 32-bit platforms that this is not safe.
• Map Agda numbers to Int64 and Word64 on 32-bit platforms. This means there won't be a mapping for Int, so many Prelude functions will be unusable.
• Implement Int32 and Word32 on the Agda side (using modular arithmetic) and map these to Int and Word on 32-bit platforms.

The last two are not mutually exclusive.