vendor/bc: import version 6.1.0

This is a production release that fixes a discrepancy from the bc
standard, a couple of memory bugs, and adds new features.

The discrepancy from the bc standard was with regards to the behavior
of the quit command. This bc used to quit whenever it encountered quit
during parsing, even if it was parsing a full file. Now, bc only quits
when encountering quit after it has executed all executable statements
up to that point.

This behavior is slightly different from GNU bc, but users will only
notice the difference if they put quit on the same line as other
statements.

The first memory bug could be reproduced by assigning a string to a
non-local variable in a function, then redefining the function with
use of the same non-local variable, which would still refer to a
string in the previous version of the function.

The second memory bug was caused by passing an array argument to the
asciify() built-in function. In certain cases, that was wrongly
allowed, and the interpreter just assumed everything was correct and
accessed memory. Now that arrays are allowed as arguments (see below),
this is not an issue.

The first feature was the addition of the is_number() built-in
function (u in dc) that returns 1 if the runtime argument is a number
and 0 otherwise.

The second feature was the addition of the is_string() built-in
function (t in dc) that returns 1 if the runtime argument is a string
and 0 otherwise.

These features were added because I realized that type-checking is
necessary now that strings can be assigned to variables in bc and
because they've always been assignable to variables in dc.

The last added feature is the ability of the asciify() built-in
function in bc to convert a full array of numbers into a string. This
means that character-by-character printing will not be necessary, and
more strings than just single-character ones will be able to be
created.