common: use explicit conversions

to avoid sign-conversion warnings. All values used in indexing and persistent_ptr
arithmetic operations are explicitly casted to uint64_t.

Based on well-defined unsinged overflow arithmetic and standard guarantees about integer
conversions:

[conv.integral]
If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source
integer (modulo 2
n where n is the number of bits used to represent the unsigned type).