serde_v8: fix pointer size assumptions

[wg]

serde_v8 cannot currently be built for 32 bit targets such as armv7-unknown-linux-musleabihf due to the code assuming pointers are 64 bits:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
   --> serde_v8/magic/transl8.rs:108:3
    |
108 |   std::mem::transmute(ptr)
    |   ^^^^^^^^^^^^^^^^^^^
    |
    = note: source type: `u64` (64 bits)
    = note: target type: `&T` (32 bits)

This PR casts the serialized u64 fields to usize before transmuting and removes the hardcoded assumption that size_of::<usize>() == 8.

[CLAassistant]

All committers have signed the CLA.

[bartlomieju]

Thanks!