add parachain c

[zqhxuyuan]

some parachain may not use GeneralKey as its token, but use just its parachain, i.e. PHA:

https://github.com/AcalaNetwork/Acala/blob/d92b2db7e384ebf24dafc89d7cc0e04ee4724435/runtime/karura/src/lib.rs#L1811-L1812

// Phala Native token
Token(PHA) => Some(MultiLocation::new(1, X1(Parachain(parachains::phala::ID)))),

this PR add parachain c, and use parachain(3) to represent its token.

[codecov-commenter]

Codecov Report

Merging #699 (f87e99f) into master (4a66b29) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   74.63%   74.74%   +0.11%     
==========================================
  Files          83       83              
  Lines        7261     7293      +32     
==========================================
+ Hits         5419     5451      +32     
  Misses       1842     1842              

Impacted Files	Coverage Δ
xtokens/src/mock/mod.rs	92.72% <100.00%> (+0.41%)	⬆️
xtokens/src/tests.rs	99.22% <100.00%> (+0.04%)	⬆️

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[shaunxw]

Parachains in mock shares the same CurrencyIdConvert, this will make all of them convert Here to CurrencyId::C, which doesn't make sense.

You'll need to define a convert impl only for Parachain C.