Rename identifiers for style consistency

Currently all identifiers are in lower_underscore_separated format,
while the entire Go standard library and most of the other existing
  projects use the lowerCamelCase format instead.

I used the following script for applying the change:
```sh
fix() {
  readonly SOURCE=${1}
  readonly TARGET=${SOURCE}_

  gsed 's/_\([a-z]\)/\u\1/g' ${SOURCE} > ${TARGET}
  mv ${TARGET} ${SOURCE}
}
export -f fix

find . -iname '*.go' -exec sh -c 'fix "{}"' \;
```

And then manually ignored a change in the `_gccgo/package.go` file
(`".go_export"`).

Feel free to apply the change yourself instead of merging the PR if you'd rather avoid my name appearing everywhere in `git blame` (I don't mind either way).