Upgraded to TS 2.4 and refactored some tests

[TMSCH]

The main change after the upgrade is the usage of the native string enums. I updated some typings related to Message and Error to better accommodate this change.

Some new errors were also raised once I updated. The Credential and CredentialRequestOptions would conflict with the CredentialsManagement's own interfaces and I think the newly added weak detection checking would start throwing errors (https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-4.html). I added a OY prefix to such types.

The other substantial change is the refactoring of the api_test test cases as they were dependent to the order, and more targeted unit tests.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.83%. The diff coverage is 88.68%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   88.28%   87.44%   -0.84%     
==========================================
  Files          34       34              
  Lines        1374     1410      +36     
  Branches      161      167       +6     
==========================================
+ Hits         1213     1233      +20     
- Misses        161      177      +16

Impacted Files	Coverage Δ
ts/protocol/enums.ts	100% <ø> (+28.57%)	:arrow_up:
ts/protocol/validators.ts	98.3% <ø> (ø)	:arrow_up:
ts/api/hint_request.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/messages.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/post_messages.ts	100% <100%> (ø)	:arrow_up:
ts/api/proxy_login.ts	100% <100%> (ø)	:arrow_up:
ts/protocol/data.ts	100% <100%> (ø)	:arrow_up:
ts/api/credential_save.ts	100% <100%> (ø)	:arrow_up:
ts/api/base_request.ts	98.52% <100%> (+0.02%)	:arrow_up:
ts/api/verify.ts	100% <100%> (ø)	:arrow_up:
... and 17 more

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[TMSCH]

Thanks Rahul!