gdb: move rust_language into rust-lang.h

Move the rust_language class declaration into the rust-lang.h header
file.  This allows for the function implementations called directly in
rust-lang.c and rust-exp.y without the need for trampoline functions.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* rust-exp.y (rust_parse): Rename to...
	(rust_language::parser): ...this.
	* rust-lang.c (-rust_printstr): Rename to...
	(rust_language::printstr): ...this.
	(rust_value_print_inner): Delete declaration.
	(val_print_struct): Rename to...
	(rust_language::val_print_struct): ...this.  Update calls to
	member functions.
	(rust_print_enum): Rename to...
	(rust_language::print_enum): ...this.  Update calls to member
	functions.
	(rust_value_print_inner): Rename to...
	(rust_language::value_print_inner): ...this.  Update calls to
	member functions.
	(exp_descriptor_rust): Rename to...
	(rust_language::exp_descriptor_tab): ...this.
	(class rust_language): Move to rust-lang.h.
	(rust_language::language_arch_info): Implementation moved to here
	from class declaration.
	(rust_language::print_type): Likewise.
	(rust_language::emitchar): Likewise.
	(rust_language::is_string_type_p): Likewise.
	* rust-lang.h: Add 'demangle.h', 'language.h', 'value.h', and
	'c-lang.h' includes.
	(rust_parse): Delete declaration.
	(class rust_language): Class declaration moved here from
	rust-lang.c.