Add definitions to b2Contact
Impl methods:
GetManifold
GetWorldManifold
SetEnabled
IsTouching
IsEnabled
GetNext
GetChildIndexA
GetChildIndexB
SetFriction
GetFriction
SetRestitution
GetRestitution
SetTangentSpeed
GetTangentSpeed
b2MixFriction
b2MixRestitution
ResetRestitution
ResetFriction
Impl methods:
AddType
Create

add classes:
b2ContactRegister
b2CircleContact

rename: assert to b2Assert