caml_alloc returns initialised blocks for tag < No_scan_tag. Otherwise,
initialise the blocks as necessary.

For Abtract_tag, Double_tag and Double_array_tag, the initial contents
are irrelevant.

Uninitialised Custom_tag objects are difficult to use correctly. Hence,
reject custom block allocations through Obj.new_block.

For String_tag, the last byte encodes the string length. Hence, reject
zero-length string objects. Initialise the last byte which encodes the
length to ensure non-negative lengths for uninitialised strings.