Hachoir core

A string value is now always Unicode, even on a
charset conversion error. Creation of field value,
display, and raw_display attributes is now fault
tolerant and uses the cache. The SeekableFieldSet
class is nearly complete and more robust.
GenericFieldSet uses a lock to avoid field
creation recursion. The limitedMemory() function
was added to call another function with a memory
limit. The Float80 type was added.