Add SHA-1 message digest

SHA-1 is an important message digest. E.g. it is used by git. This implementation features:
- optimized standard implementation. E.g. if compiled with -O -inline -release then there is not single function call or loop.
- if SSSE3 support is detected then a special assembler function is used. This gives a speedup of 1.4 on 32 bit and about 1.8 on 64 bit
The 64 bit implementation is a bit clumsy because of some issues with the dmd compiler (issues 6459 and 5355).
Supports 32 bit Windows and 32 bit and 64 Linux. MacOS X, FreeBSD are untested.

posix.mak

@@ -160,7 +160,7 @@ STD_MODULES = $(addprefix std/, algorithm array ascii base64 bigint		\
         encoding exception file format functional getopt gregorian		\
         json loader math mathspecial md5 metastrings mmfile numeric		\
         outbuffer parallelism path perf process random range regex		\
-        regexp signals socket socketstream stdint stdio stdiobase		\
+        regexp sha1 signals socket socketstream stdint stdio stdiobase	\
         stream string syserror system traits typecons typetuple uni		\
         uri utf variant xml zip zlib)
 
@@ -182,7 +182,9 @@ endif
 EXTRA_DOCUMENTABLES += $(addprefix etc/c/,curl zlib) $(addprefix		\
 std/c/, fenv locale math process stdarg stddef stdio stdlib string	\
 time wcharh)
-EXTRA_MODULES += $(EXTRA_DOCUMENTABLES) $(addprefix			\
+EXTRA_MODULES_DIGEST += $(EXTRA_DOCUMENTABLES) $(addprefix			\
+	std/internal/digest/, sha1_SSSE3)
+EXTRA_MODULES += $(EXTRA_MODULES_DIGEST) $(addprefix			\
 	std/internal/math/, biguintcore biguintnoasm biguintx86	\
 	gammafunction errorfunction)