Fixed|libdeng1|Reader|Writer: Avoiding pointer alignment issues

Some CPU architectures do not look kindly upon pointer misalignment.
Fixed|libdeng1|Reader: Possibly undefined operation

Attempting to fix a warning about post-increments.
Fixed|libdeng1|Reader: Reading 16/32-bit values

An incorrect return type for Reader_8() caused the higher bytes of
16/32-bit values to be truncated to zero.