Replace function/method type symbol => with ->

The `->` symbol is more consistent with existing languages, and distinguishes the first-class types of functions from maps. This purely syntactic change modifies the lexers, parsers, and writers of `whiley` and `wyal` files.

The remaining changes are in either code documentation or tests and expected test outputs. These changes are largely automatic: the changes to Java files have been verified by hand, and running the tests have verified the changes to the test files.

Two existing errors in documentation on pointer property access have also been corrected.

As with the line-ending changes, this commit is relatively automatic but changes a large number of files. Merge with care.