Improve handling of strings in QML cache files

Strings in QML cache files should not be copied but merely referenced and we should not unmap the cache files anymore.

Potential side-/sub-tasks include distinguishing on the JS side between run-time strings and read-only strings (different v4 string/vtable) and generating the full QArrayData so that we don't even need to allocate the the "string private" anymore.

Preliminary analysis suggests that that 64-bit version of the array data for a read-only string may be binary compatible with the 32-bit version, on little-endian.

For the V4 value string handling, perhaps we can store a pointer to the array data and tag it accordingly, so that a V4 value pointing to such a read-only string is free of any gc or allocation overhead.

The read-only QString embedding in the cache files gives ~5% progression on creation/qml.basictypes/delegates_basictype_string.qml and ~3% progression on creation/qml.modelaccess/modelaccess_string.qml on linux x86-64 (release).