lib: rename stdlib surface to Hare names; add endian/math

Sweeping rename so the lib/ surface mirrors Hare's stdlib spellings.
- ascii: rune-taking predicates; ishex -> isxdigit
- bufio: rinit -> init; take1/takeline -> readbyte/readline
- bytes: indexsub -> index
- encoding/utf8: runelen -> runesz
- errors: eEOF/eShortRead/... -> eof/underread/...
- fmt: errln -> errorln; println/fprintln return i64
- os: readfull/writefull -> readall/writeall; unlink -> remove
- path: isabs -> abs; drop lastindex (now strings.rbyteindex)
- strconv: u64toa/i64toa -> u64tos/i64tos; parse64/parseu64 -> stoi64/stou64
- strings: drop len/isempty; equal -> compare; indexbyte -> byteindex; +rbyteindex
- types: drop numeric helpers (moved to math)
- new lib/endian (htonu16/ntohu16), lib/math (absi32/absi64)
- net: drop htons (use endian.htonu16)

Callers in selfhost/, lib/ww/, cmd/w6c/cgen.c, and test/wcc/700_e2e.c
updated to match.

lib/errors/errors.ww

@@ -3,14 +3,14 @@
 
 type error = str;
 
-def eEOF:        error = "eof";
-def eShortRead:  error = "short read";
-def eShortWrite: error = "short write";
-def eClosed:     error = "closed";
-def eInvalid:    error = "invalid argument";
-def ePerm:       error = "permission denied";
-def eNotFound:   error = "not found";
-def eExists:     error = "already exists";
+def eof:        error = "eof";
+def underread:  error = "short read";
+def underwrite: error = "short write";
+def closed:     error = "closed";
+def invalid:    error = "invalid argument";
+def noaccess:   error = "permission denied";
+def noentry:    error = "not found";
+def exists:     error = "already exists";
 
 export fn isnil(e: error) bool = {
 	return e.len == 0;