Default strings and vectors: Parser + Rust support

[CasperN]

First steps towards #6053:

• Strings can have any string as a default
• vectors can only have the empty vector as a default

This should go in after #6420

[aardappel]

Please rebase.

[CasperN]

Rebased and added more tests. @aardappel, PTAL again

[aardappel]

This generally looks good! Though not sure if this should go in without at least 1 language that uses this data.. I understand you're wanting to keep commits small, but it also has value to see new data actually used.

[aardappel]

Can you undo the unrelated clang-format changes?

Are you using our script, or at least git-clang-format ? We typically only want changed code to be formatted, not the whole file.

[aardappel]

This looks great.. ready to merge, unless you want anyone else to review it.

[CasperN]

@krojew, can I also get your review?

[vglavnyy]

@CasperN
After this commit this test is failed:

  using flatbuffers::Parser;
  TEST_EQ(false, Parser().Parse("enum e:e{}table T{p:[e]=[]"));

with assertion:

Assertion failed: field->value.constant == "0", file E:\github\flatbuffers\src\idl_parser.cpp, line 917

Could you take a look at this problem?

[aardappel]

Should probably be fixed before #6353

[CasperN]

dang it, I made the same mistake while doing optional scalars 🙄