Updated parsing of typedef enclosed declarations

[mannprerak2]

• Added getCursorUSR_wrap to wrapper.c, updated wrapper version to v3,
• Updated code to use USR obtained via libclang to uniquely identify declarations. This prevents skipping of struct and typedefs with similar names (closes Struct and typedef Struct name conflicts dart-lang/native#403).
• Name from the global namespace is used (affects structs and enums as they can be inside a typedef) (closes [Question] typedef struct dart-lang/native#400).
• If declaration is excluded according to config the above rule isn't followed and the name used in the first function parsed is used.
• Added tests, updated test_coverage.dart.

Example -

Suppose we have a struct _S(struct namespace) enclosed in typedef S(global namespace), some typerefs and a function using struct via a typeref.

typedef struct _S{

} S; // S is in global namespace, _S is in struct namespace.

// These typerefs are not considered.
typedef S S1;
typedef S S2;
typedef S S3;

void func(S1*);

Ffigen follows the following rules

• If only _S or S is included according to config, that particular name is used.
• If both _S and S are included according to config, name in the global namespace, S is used.
• If both _S and S are excluded according to config, the name used by the first function parsed (S1` in the example) will be used.