Replace identifiers and class names if they happen to be keywords

Some identifiers are legal in C, but not in Java. An example is a
function parameter called "native". Add a leading underscore to
these to solve the problem.

Native function names are not handled this way in this commit.
Temporarily disable debug XML comment

The contents of verbatim snippets from the XMLs added as comments to
generated methods cause compilation problems with illegal Unicode
escapes. Disable them temporarily while getting the generated code
to compile.
Prefix enum constant names with class name

Some enum constant names defined in .gir files are not legal Java,
for example, they can begin with a number. The c:identifier is not
always usable either, as in some cases it contains a free-form
description. Transforming the enum class name from CamelCase to
ALL_UPPERCASE_WITH_UNDERSCORES_, and prefixing it to the enum
constant names should produce legal Java enum constant names.
Prefix record/class field getters and setters with field_

Some method names in .gir files conflict with field names in their
classes or records. With the current generation scheme, both methods,
and fields are bound to methods in their classes, named after the
name declared in the .gir files. This may lead to the generation of
multiple methods with the same signature in the same class. Solve
this by prefixing field getters and setters with field_.
Do not use return in pretty wrappers returning void