Reduce the memory usage of terms

[mlaveaux]

Currently the SharedTerm is way too large, requiring 2 words for the ptr and size in the vector of arguments and 2 words for the annotations. The separate allocation of ptr also has 2 words of overhead. This should be eliminated before comparing it to the mCRL2 toolset.

[mlaveaux]

The annotation overhead has been resolved by using (an unsafe) union of an annotation and aterm as argument. This currently still requires storing a boolean, but that could be done in the function symbol in the future.

[mlaveaux]

The vector has been replaced by making SharedTerm a dynamic sized type, but this still requires a fat pointer to store the length (at least the capacity is gone). Furthermore, some progress has been made on the block allocator, which would avoid the allocation overhead.

[mlaveaux]

The fat pointer has been avoided (I think), but now the type is no longer FFI safe.