Move nonstring and memory APIs from lib/string/ to a new lib/memory/

[alejandro-colomar]

@ikerexxe It turns out it was less work than I thought, so we can do the movement already. Here it is.

See also memory.h(3head): https://www.kernel.org/pub/linux/docs/man-pages/book/man-pages-6.19.pdf#memory.h.3head.

---

Revisions:

v1b

• Update paragraph in the README. [@ikerexxe ]

$ git rd 
 1:  f65d2fe1b947 =  1:  f65d2fe1b947 lib/memory/README: Add symlink to the string README
 2:  a686e1b76ef7 !  2:  81219686b3fb lib/string/README: Reorganize the library in memory/ and string/
    @@ Commit message
         Signed-off-by: Alejandro Colomar <alx@kernel.org>
     
      ## lib/string/README ##
    -@@ lib/string/README: Specific guidelines:
    -   strings, separated in subdirectories by utility type.  In this
    -   section, we provide a broad overview.
    +@@ lib/string/README: Don't use some libc functions without Really Good Reasons:
    + 
    + Specific guidelines:
    + ====================
    ++  Under lib/memory/ we provide a set of functions to manipulate
    ++  bytes --mem*()-- and nonstrings --strn*()--.
    +   Under lib/string/ we provide a set of functions to manipulate
    +-  strings, separated in subdirectories by utility type.  In this
    +-  section, we provide a broad overview.
    ++  strings.
    ++  All of these functions are separated in subdirectories
    ++  by utility type.
    ++  In this section, we provide a broad overview of all of them.
      
     -ctype/ - Character classification and conversion functions
     +memory/ - memory operations
 3:  c3612a7db1b8 =  3:  8cdf0ce71aae lib/string/README: Remove strndup_a()
 4:  6757c8c928a5 =  4:  0943230b4d4f lib/string/README: Rename MEMCPY() => memcpy_a()
 5:  1249699267a0 =  5:  cb653ae17c53 lib/string/README: Document strncmp(3) and strncat(3) as being from <memory.h>
 6:  51efa2ddff9d =  6:  2312c3889cf0 lib/, src/: Move memzero() to under lib/memory/
 7:  d421d1236e18 =  7:  0ded15960a1c lib/, src/: Move strzero() to under lib/string/strzero/
 8:  99bf4380d941 =  8:  886f51695d59 lib/: Move memdup() and strndup() under lib/memory/
 9:  77990d3a2ab2 =  9:  5c627cb5a4c1 lib/, src/: Move strneq() to under lib/memory/
10:  8db526f99e40 = 10:  459e7d946156 lib/: Move strncpy/cat() to under lib/memory/

[ikerexxe]

One minor comment inline, apart from that this looks good

I'd also like to hear @hallyn's opinion before merging