[Refactor] nvm_alias, nvm_resolve_alias: use builtins

[Tests] `nvm_alias`, `nvm_resolve_alias`: add edge-case tests

nvm_alias() used a sed/awk pipeline to strip comments and blank lines from alias files that almost always contain a single word.
A while-read loop with parameter expansion does the same filtering more directly.

nvm_resolve_alias() piped nvm_alias through head and tail to extract one line, and used printf/grep for cycle detection.
Parameter expansion and a case statement replace both without the extra plumbing.

All replacements are POSIX (read -r, case, IFS=, parameter expansion).
As a side effect, this also removes 4 external process invocations during shell init.

[Fix] `nvm_resolve_alias`: detect cycles via newline-anchored `case`

The original commit referenced above changed SEEN_ALIASES from `\n`-delimited
storage (interpreted by `printf '%b' | nvm_grep -e "^${name}$"`) to space-
delimited but left the line-anchored grep in place — without newlines in
the haystack the anchored pattern can never match, so cycles never break.

Switch to literal-newline storage and a `case` pattern anchored on those
newlines. Newline anchoring also handles alias names containing spaces,
which token-based patterns false-positive on (e.g. lookup of `bar` matches
substring " bar " inside " foo bar midway " when the chain visits the
multi-token alias `foo bar`).

New test file covers self-loop, multi-hop loop, cycle through a
space-bearing alias name, and a non-cycle through a space-bearing
alias name. Existing `test/fast/Aliases/circular/` fixtures continue
to pass.

test/fast/Aliases/nvm_resolve_alias with nonexistent target

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+export NVM_DIR="$(cd ../../.. && pwd)"
+
+: nvm.sh
+\. "${NVM_DIR}/nvm.sh"
+
+# Create an alias pointing to a version that does not exist as an alias
+echo '99.99.99' > ../../../alias/test-edge-noexist
+
+ACTUAL="$(nvm_resolve_alias test-edge-noexist)"
+EXPECTED='v99.99.99'
+EXIT_CODE="$(nvm_resolve_alias test-edge-noexist >/dev/null 2>&1; echo $?)"
+[ "${ACTUAL}" = "${EXPECTED}" ] || die "expected >${EXPECTED}<, got >${ACTUAL}<"
+[ "${EXIT_CODE}" = '0' ] || die "expected exit code 0, got ${EXIT_CODE}"
+
+rm -f ../../../alias/test-edge-noexist