Log Message:
rgbds: update to 0.5.1

   Added:

     * RGBFIX now supports the homebrew TPP1 mapper! (The emulators BGB 1.5.9
       and SameBoy 0.14.3 do too)
     * New \<10> and \<num_sym> macro arg syntaxes
     * SIZEOF("section") and STARTOF("section")
     * CHARLEN and CHARSUB for working with charmap output units
     * OPT L, OPT W, and OPT h
     * REDEF EQU
     * Linker scripts can use character escapes in section names ("\n \r \t
       \" \\")
     * RGBFIX now reports when it's overwriting non-zero header bytes,
       helpful for spotting mistakes (thanks @GreenAndEievui!)
     * rgbfix -m help will list the accepted MBC names
     * rgbfix -m accepts $-prefixed hex literals

   Changed:

     * Parentheses in macro args implicitly escape commas now, like C. For
       example, macro STRCAT("foo", "bar") used to be 2 \ 
arguments, now it's
       only one!
     * STRSUB and CHARSUB allow negative start positions, relative to the end
       of the string
     * STRSUB's second parameter can be omitted to read until the end of the
       string
     * A lot more errors are non-fatal now
     * Any constant expression equal for $FF00 can be substituted to $ff00 in
       ld [$ff00 + c], a and ld a, [$ff00 + c]; and arbitrary whitespace,
       line continuations, and block comments are allowed

   Removed:

     * LOAD FRAGMENT had buggy behavior, and its use cases can be done
       differently

   Fixed:

     * Symbols in some SECTION UNION/FRAGMENT could be missing from the .sym
       file
     * Interpolations could recurse infinitely outside of strings
     * ld hl, sp - <expr> was broken in 0.5.0 (workaround: ld hl, sp +
       -<expr>)
     * $ff00 + constant (or any symbol name starting with "c") would \ 
cause a
       syntax error since 0.4.2 (workarounds: ($ff00) + constant, constant +
       $ff00, or $ff00 + /**/ constant)
     * Fixed a lexer bug that could crash RGBASM on "piped" input files
     * jr could be incorrect within SECTION FRAGMENT
     * PUSHS didn't reset the section scope correctly (regression), and
       didn't save the LOAD state
     * REDEF EQUS had a few bugs
     * RGBFIX incorrectly parsed options on some platforms
     * RGBLINK should no longer leak memory
     * Fixed some compiler warnings, notably on 32-bit platforms and macOS
     * The source now uses _ISO_C11_SOURCE; this notably fixes building on
       FreeBSD (see #789 for more info)