Change target_write_memory_blocks to use std::vector

Change target_write_memory_blocks to use std::vector

This changes target_write_memory_blocks to use std::vector, rather
than VEC.  This allows the removal of some cleanups.

This version incorporates the additions that Simon made.

Regression tested by the buildbot.

ChangeLog
2018-02-27  Simon Marchi  <simon.marchi@polymtl.ca>
	    Tom Tromey  <tom@tromey.com>

	* target.h (memory_write_request_s): Remove typedef.  Don't define
	VEC.
	(target_write_memory_blocks): Change argument to std::vector.
	(struct memory_write_request): Add constructor.
	* target-memory.c (compare_block_starting_address): Return bool.
	Change argument types.
	(claim_memory): Change arguments to use std::vector.
	(split_regular_and_flash_blocks, blocks_to_erase)
	(compute_garbled_blocks): Likewise.
	(cleanup_request_data, cleanup_write_requests_vector): Remove.
	(target_write_memory_blocks): Change argument to std::vector.
	* symfile.c (struct load_section_data): Add constructor and
	destructor.  Use std::vector for "requests".
	(struct load_progress_data): Add initializers.
	(load_section_callback): Update.  Use "new".
	(clear_memory_write_data): Remove.
	(generic_load): Update.