univalue: Avoid unnecessary roundtrip through double for numbers

JSON makes no distinction between numbers and reals, and our code
doesn't need to do so either.

This removes VREAL, as well as its specific post-processing in
`UniValue::write`. Non-monetary amounts do not need to be forcibly
formatted with 8 decimals, so the extra roundtrip was unnecessary
(and potentially loses precision).

Zcash: cherry-picked from commit 7650449a6777710cf818d41862626164da0cd412
Left over from 2aee461930

qa/rpc-tests/rest.py

@@ -14,6 +14,7 @@ from struct import *
 import binascii
 import json
 import StringIO
+import decimal
 
 try:
     import http.client as httplib
@@ -252,7 +253,7 @@ class RESTTest (BitcoinTestFramework):
         response_header_json = http_get_call(url.hostname, url.port, '/rest/headers/1/'+bb_hash+self.FORMAT_SEPARATOR+"json", "", True)
         assert_equal(response_header_json.status, 200)
         response_header_json_str = response_header_json.read()
-        json_obj = json.loads(response_header_json_str)
+        json_obj = json.loads(response_header_json_str, parse_float=decimal.Decimal)
         assert_equal(len(json_obj), 1) #ensure that there is one header in the json response
         assert_equal(json_obj[0]['hash'], bb_hash) #request/response hash should be the same