bind: fix Objective-C code for struct field name with uppercase prefix

[xudongzheng]

Objective-C code generated by gobind does not correct set a struct field whose name has an upper case prefix. This includes struct fields with names such as URL, ID, etc.

Consider the following Go code:

package example

type S struct {
        BRAVO string
}

func Test(s *S) string {
        return s.BRAVO
}

Generate bindings with gomobile bind -target ios example.

Use the following code for iOS:

let s = ExampleS()
s.bravo = "example"
print("result", ExampleTest(s))

The output is "result" rather than "result example".

The current gobind generates:

- (void)setBRAVO:(NSString* _Nonnull)v {
        int32_t refnum = go_seq_go_to_refnum(self._ref);
        nstring _v = go_seq_from_objc_string(v);
        proxyexample_S_BRAVO_Set(refnum, _v);
}

It should instead generate

- (void)setBravo:(NSString* _Nonnull)v {
        int32_t refnum = go_seq_go_to_refnum(self._ref);
        nstring _v = go_seq_from_objc_string(v);
        proxyexample_S_BRAVO_Set(refnum, _v);
}

This pull request fixes this for struct fields. This also makes the change for package's top level variables for consistency.

The same problem doesn't exist for Java/Android.

[gopherbot]

This PR (HEAD: 9935c16) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/mobile/+/240009 to see it.

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