Backport of WL #2934: Make/find library for doing float/double

to string conversions and vice versa" 
Initial import of the dtoa.c code and custom wrappers around it 
to allow its usage from the server code. 
 
Conversion of FLOAT/DOUBLE values to DECIMAL ones or strings 
and vice versa has been significantly reworked. As the new 
algoritms are more precise than the older ones, results of such 
conversions may not always match those obtained from older 
server versions. This in turn may break compatibility for some 
applications. 
 
This patch also fixes the following bugs: 
- bug #12860 "Difference in zero padding of exponent between 
Unix and Windows" 
- bug #21497 "DOUBLE truncated to unusable value" 
- bug #26788 "mysqld (debug) aborts when inserting specific 
numbers into char fields" 
- bug #24541 "Data truncated..." on decimal type columns 
without any good reason"

mysql-test/r/func_str.result

@@ -1354,10 +1354,10 @@ cast(rtrim(ltrim('  20.06 ')) as decimal(19,2))
 20.06
 select conv("18383815659218730760",10,10) + 0;
 conv("18383815659218730760",10,10) + 0
-1.83838156592187e+19
+1.838381565921873e19
 select "18383815659218730760" + 0;
 "18383815659218730760" + 0
-1.83838156592187e+19
+1.838381565921873e19
 CREATE TABLE t1 (code varchar(10));
 INSERT INTO t1 VALUES ('a12'), ('A12'), ('a13');
 SELECT ASCII(code), code FROM t1 WHERE code='A12';