[swift2objc] Support optionals

[liamappelbe]

Optionals in Swift are essentially the same as nullables in Dart. This PR adds support for them by parsing them into the new OptionalType class.

The parsing side of this is pretty complicated, because unlike classes, structs, and primitives, optional types aren't presented in the symbolgraph as a single token with a type ID. Instead we get a "?" token that comes after the type token. So now we have to write a parser.

I wrote a pretty general parser architecture, because we're also going to need it for closures, tuples, and arrays, which also appear as a sequence of tokens. I used a Pratt parser, but without any precedence, so it ends up being pretty simple and readable. The entire parser is in parse_type.dart, and the entrypoint is parseType.

One extra complexity is that sometimes some of the tokens are concatenated. For example, you'd expect foo(bar: Int?) to be tokenized as ["foo", "(", "bar", ": ", "Int", "?", ")"], but actually the "?" and ")" get merged: ["foo", "(", "bar", ": ", "Int", "?)"]. So we also need a preprocessing stage that splits up all the known cases where this happens. This is handled by the TokenList class.

Other changes

• Had to rewrite parseInitializerParams to use the new parseType function, but it ends up being simpler and more readable.
• Removed id and name from ReferredType, since not all types have a name or ID.
• All the type comparisons that used to be id based have been migrated to the new sameAs method.
• All the codegen that used to print the name now prints the swiftType.
• Made FunctionDeclaration's returnType non-nullable. Not specifying the return type of a function just means it's Void in Swift, so there's no point making this field nullable. Making it non-null simplifies some stuff.
• Renamed pase_function_declaration.dart to parse_function_declaration.dart
• Removed test/unit/parse_initializer_param_input.json because it isn't actually used by the test.

Optional primitives aren't supported atm: #1743

[coveralls]

coverage: 90.171% (-0.03%) from 90.198%
when pulling c34d60e on optionals
into 7afad0b on main.

[HosseinYousefi]

nit: Wrap code with backticks. [] might do some linking stuff in markdown otherwise.

[liamappelbe]

Done