Re-order members of MVMCompUnitBody to avoid alignment holes.

Saves 4 bytes on ARM and x86, 32 bytes on x86_64.
Re-order members of MVMSerializationRoot to avoid alignment holes.

Saves 16 bytes on x86_64.
Re-order members of MVMThreadContext to avoid alignment holes.

Saves 8 bytes on ARM, and 16 bytes on x86_64.
Re-order members of MVMSpeshBB to avoid alignment holes.

Saves 8 bytes on ARM and x86, 24 bytes on x86_64.
Re-order members of MVMStaticFrameBody to avoid alignment holes.

Saves 8 bytes on x86_64.
Re-order members of MVMP6opaqueREPRData to avoid alignment holes.

Saves 4 bytes on ARM and x86, 16 bytes on x86_64.
Re-order members of MVMInstance to avoid alignment holes.

Saves 4 bytes on ARM, and 8 bytes on x86_64.
Re-order members of MVMSpeshCandidate to avoid alignment holes.

Saves 8 bytes on x86_64.
Re-order members of MVMSTable to avoid alignment holes.

Saves 8 bytes on x86_64.
Re-order members of MVMSerializationContextBody to avoid alignment holes.

Saves 4 bytes on ARM.
Re-order members of MVMInvocationSpec to avoid alignment holes.

Saves 4 bytes on ARM.
Add ChangeLog entry about struct re-arrangement.