feat: Replaced generated code with stellar-xdr

[bjorn]

Removed the dependencies xdr-rs-serialize and xdr-rs-serialize-derive, since serialization is already provided by stellar-xdr.

For compatibility reasons and convenience, the existing XDRSerialize and XDRDeserialize traits are now implemented on top of the WriteXdr and ReadXdr traits from stellar-xdr.

Added a bunch of new data types and enum values. Removed enum types where they added nothing beyond the same type in stellar-xdr. Not all new types have been wrapped. For example, the inner data of the new LedgerKey values instead refers directly to the types from the stellar-xdr crate. This can be changed when there is demand, but it seems futile to wrap all these types in verbose API and to keep this updated.

No support was added for Preconditions::V2 and TransactionExt::V1, both part of the new stellar_xdr_curr::Transaction type. When trying to convert transactions which use these values an UnsupportedFeature error is returned for now.

Closes #10

[bjorn]

@fracek I think I'm going a little crazy doing this change.

[bjorn]

OMG, it compiles and all tests pass!

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 209 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.53%. Comparing base (98f7202) to head (4f4279d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/operations/invoke_host_function.rs	45.61%	31 Missing ⚠️
src/transaction.rs	51.61%	30 Missing ⚠️
src/claim.rs	25.00%	24 Missing ⚠️
src/operation_result.rs	78.84%	22 Missing ⚠️
src/crypto/signature.rs	63.82%	17 Missing ⚠️
src/asset.rs	60.52%	15 Missing ⚠️
src/transaction_result.rs	42.30%	15 Missing ⚠️
src/operations/revoke_sponsorship.rs	39.13%	14 Missing ⚠️
src/operations/change_trust.rs	43.75%	9 Missing ⚠️
src/operations/mod.rs	70.96%	9 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   65.77%   65.53%   -0.24%     
==========================================
  Files          43       45       +2     
  Lines        3795     3839      +44     
==========================================
+ Hits         2496     2516      +20     
- Misses       1299     1323      +24     

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