Rewrite RGBLINK entirely

The goal was to improve readability, but along the way a few things were
gained.
- Sorted sym and map files
- Infrastructure for supporting multiple .o versions
- Valgrind-proof, as far as my testing goes anyways
- Improved verbosity messages
- Added error checking
- Performance improvements, see end of commit message

The readability improvement was spurred while trying to make sense of the
old code while trying to implement features such as sorted sym and map
files.
I also did my best to remove hardcoded logic, such that modifications
should be doable; for example, "RAM loading" sections, which are linked
against a different location than the one they're stored at.

Some work remains to be done, see the "TODO:" and "FIXME:" comments.
Further, while regression tests pass, this new linker should be tested on
different codebases (ideally while instrumented with `make develop` and
under valgrind).
The few errors spotted in the man pages (alignment) need to be corrected.
Finally, documentation comments need to be written, I have written a lot of
them but not all.

This also provides a significant performance boost (benchmarked with a
51994-symbol project):

Current master RGBLINK:
2.02user 0.03system 0:02.06elapsed 99%CPU (0avgtext+0avgdata 84336maxresident)k
0inputs+11584outputs (0major+20729minor)pagefaults 0swaps

Rewritten RGBLINK:
0.19user 0.06system 0:00.63elapsed 40%CPU (0avgtext+0avgdata 32460maxresident)k
23784inputs+11576outputs (0major+7672minor)pagefaults 0swaps

include/link/section.h

@@ -0,0 +1,112 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 1997-2019, Carsten Sorensen and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/* Declarations manipulating symbols */
+#ifndef RGBDS_LINK_SECTION_H
+#define RGBDS_LINK_SECTION_H
+
+/* GUIDELINE: external code MUST NOT BE AWARE of the data structure used!! */
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#include "link/main.h"
+
+#include "linkdefs.h"
+
+struct AttachedSymbol {
+	struct Symbol *symbol;
+	struct AttachedSymbol *next;
+};
+
+struct Patch {
+	char *fileName;
+	int32_t lineNo;
+	int32_t offset;
+	enum PatchType type;
+	int32_t rpnSize;
+	uint8_t *rpnExpression;
+};
+
+struct Section {
+	/* Info contained in the object files */
+	char *name;
+	uint16_t size;
+	enum SectionType type;
+	bool isAddressFixed;
+	uint16_t org;
+	bool isBankFixed;
+	uint32_t bank;
+	bool isAlignFixed;
+	uint16_t alignMask;
+	uint8_t *data; /* Array of size `size`*/
+	uint32_t nbPatches;
+	struct Patch *patches;
+	/* Extra info computed during linking */
+	struct Symbol **fileSymbols;
+	uint32_t nbSymbols;
+	struct Symbol const **symbols;
+};
+
+extern uint16_t startaddr[];
+extern uint16_t maxsize[];
+extern uint32_t bankranges[][2];
+extern char const * const typeNames[SECTTYPE_INVALID];
+
+/**
+ * Computes a memory region's end address (last byte), eg. 0x7FFF
+ * @return The address of the last byte in that memory region
+ */
+static inline uint16_t endaddr(enum SectionType type)
+{
+	return startaddr[type] + maxsize[type] - 1;
+}
+
+/**
+ * Computes a memory region's number of banks
+ * @return The number of banks, 1 for regions without banking
+ */
+static inline uint32_t nbbanks(enum SectionType type)
+{
+	return bankranges[type][1] - bankranges[type][0] + 1;
+}
+
+/*
+ * Execute a callback for each section currently registered.
+ * This is to avoid exposing the data structure in which sections are stored.
+ * @param callback The function to call for each structure;
+ *                 the first argument will be a pointer to the structure,
+ *                 the second argument will be the pointer `arg`.
+ * @param arg A pointer which will be passed to all calls to `callback`.
+ */
+void sect_ForEach(void (*callback)(struct Section *, void *), void *arg);
+
+/**
+ * Registers a section to be processed.
+ * @param section The section to register.
+ */
+void sect_AddSection(struct Section *section);
+
+/**
+ * Finds a section by its name.
+ * @param name The name of the section to look for
+ * @return A pointer to the section, or NULL if it wasn't found
+ */
+struct Section *sect_GetSection(char const *name);
+
+/**
+ * `free`s all section memory that was allocated.
+ */
+void sect_CleanupSections(void);
+
+/**
+ * Checks if all sections meet reasonable criteria, such as max size
+ */
+void sect_DoSanityChecks(void);
+
+#endif /* RGBDS_LINK_SECTION_H */