Large object layouts don't fit in a pointer-sized integer and therefore
need to be stored in a global instead. However, the way the data was
stored in these globals was not correct for buffers that don't have
pointers near the end. This commit fixes this issue by using math/big
FillBytes() instead of Bytes().
This gets the unicode package to compile on AVR.
This commit adds object layout information to new heap allocations. It
is not yet used anywhere: the next commit will make use of it.
Object layout information will eventually be used for a (mostly) precise
garbage collector. This is what the data is made for. However, it is
also useful in the interp package which can work better if it knows the
memory layout and thus the approximate LLVM type of heap-allocated
objects.
