bug#6891: `string-to-number' doesn't recognize `#'' read syntax

[MON KEY]

`string-to-number' takes an optional arg BASE but doesn't recognize lisp `#'
read syntax for binary, octal, hex, and radix, e.g.  `#o', `#b', `#x', and
`#<RADIX>r'. It should, these _are_ the respective read syntax for BASE.
For good measure it should also recognize the string output from
`format's `#' flag for octal and hex e.g `0x' for hex and `0' for octal.

The proposed behaviour should not pose a lisp dialect compatiblility problem as
Emacs lisp does not attempt to follow the Common Lisp spec, does not maintain a
duplicate for CL's `parse-integer' procedure and does not take a bounding START
END arg, but _does_ parse floats so it shouldn't be restricted by that specs
constraints w/re the `#' radix read syntax e.g:

,----
| `parse-integer' does not recognize the syntactic radix-specifier
| prefixes `#O', `#B', `#X', and `#nR', nor does it recognize a trailing
| decimal point.
`----

Besides, `string-to-number' already has a BASE argument, the C code already
strips whitespace, discards "junk", and exhibits a willingness to interpolate
sign in sometimes odd ways, so a little more DWIM can't hurt.

Current `string-to-number' DWIMness:
string-to-number "   -3777777777" 8) ;=> 536870911
(string-to-number "   -3777777777.007" 8) ;=> 536870911
(string-to-number "   -3777777777mmm#" 8) ;=> 536870911
(string-to-number "+   -3777777777mmm#" 8) ;=> 0

Following illustrates the additioanl DWIMing desired:

#o
(string-to-number "#o3777777777" 8) ;=> 536870911

#b
(string-to-number "#b11111111111111111111111111111" 2) ;=> 536870911

#x
(string-to-number "#x1fffffff" 16) ;=> 536870911

#<RDX>r
(string-to-number "#12r12B969227" 12) ;=> 536870911

0x
(string-to-number "0x1fffffff" 16) ;=> 536870911

0
(string-to-number "03777777777" 8) ;=> 536870911


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