Add a filter for filtering unsupported MultiLocation

[Dengjianping]

Closes #714

Add an associated type MultiLocationsFilter to filter unsupported MultiLocation.

[codecov-commenter]

Codecov Report

Merging #714 (8ff5aa1) into master (9e041dc) will increase coverage by 0.08%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage   75.37%   75.46%   +0.08%     
==========================================
  Files          83       83              
  Lines        7441     7476      +35     
==========================================
+ Hits         5609     5642      +33     
- Misses       1832     1834       +2     

Impacted Files	Coverage Δ
xtokens/src/mock/para.rs	75.75% <ø> (ø)
xtokens/src/lib.rs	64.68% <83.33%> (+0.72%)	⬆️
xtokens/src/tests.rs	99.37% <100.00%> (+0.02%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[xlc]

more test with transfer_multiassets and transfer_multicurrencies etc

[zqhxuyuan]

we could offer some default implementation(in trait/location.rs), and use Tuple combine with fall-through way. i.e. AllowParentAccount, AllowParachainAccount, AllowLocalAccount. so parachain can use like type MultiLocationsFilter = (AllowParentAccount, AllowParachainAccount) or type MultiLocationsFilter = (AllowParentAccount, AllowParachainAccount, AllowLocalAccount) if they allow local asset.

[zqhxuyuan]

or a simple way like:

match_type! {
	pub type ParentOrParachain: impl Contains<MultiLocation> = {
		MultiLocation { parents: 1, interior: X1(AccountId32(_)) } |
		MultiLocation { parents: 1, interior: X2(Parachain(_), AccountId32(_)) }
	};
}

if allow local asset:

match_type! {
	pub type ParentOrParachainOrLocal: impl Contains<MultiLocation> = {
                 MultiLocation { parents: 0, interior: X1(AccountId32(_)) } |
		MultiLocation { parents: 1, interior: X1(AccountId32(_)) } |
		MultiLocation { parents: 1, interior: X2(Parachain(_), AccountId32(_)) }
	};
}

[Dengjianping]

I cannot use Parachain(_), becasue it will match all para id.