Move expansion of strings in binaries to v3_core

This speeds up the compilation of binary literals
with string values in them. For example, compiling
a file with a ~340kB binary would yield the following
times by the compiler:

    Compiling "foo"
     parse_module       :   0.130 s    5327.6 kB
     transform_module   :   0.000 s    5327.6 kB
     lint_module        :   0.011 s    5327.8 kB
     expand_module      :   0.508 s   71881.2 kB
     v3_core            :   0.463 s      11.5 kB

Notice the increase in memory and processing time
in expand_module and v3_core. This happened because
expand_module would expand the string in binaries
into chars. For example, the binary <<"foo">>, which
is represented as

    {bin, 1, [
      {bin_element, 1, {string, 1, "foo"}, default, default}
    ]}

would be converted to

    {bin, 1, [
      {bin_element, 1, {char, 1, $f}, default, default},
      {bin_element, 1, {char, 1, $o}, default, default},
      {bin_element, 1, {char, 1, $o}, default, default}
    ]}

However, v3_core would then traverse all of those
characters and convert it into an actual binary, as it
is a literal value.

This patch addresses this issue by moving the expansion
of string into chars to v3_core and only if a literal
value cannot not be built. This reduces the compilation
time of the file mentioned above to the values below:

    Compiling "bar"
     parse_module       :   0.134 s    5327.6 kB
     transform_module   :   0.000 s    5327.6 kB
     lint_module        :   0.005 s    5327.8 kB
     expand_module      :   0.000 s    5328.7 kB
     v3_core            :   0.013 s      11.2 kB

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/compiler/test/bs_utf_SUITE.erl b/lib/compiler/test/bs_utf_SUITE.erl
index c894041f72..ef3fc54b37 100644
--- a/lib/compiler/test/bs_utf_SUITE.erl
+++ b/lib/compiler/test/bs_utf_SUITE.erl
@@ -235,6 +235,7 @@ utf32_to_unicode(<<>>) -> [].
 literals(Config) when is_list(Config) ->
     abc_utf8 = match_literal(<<"abc"/utf8>>),
     abc_utf8 = match_literal(<<$a,$b,$c>>),
+    abc_utf8 = match_literal(<<$a/utf8,$b/utf8,$c/utf8>>),
 
     abc_utf16be = match_literal(<<"abc"/utf16>>),
     abc_utf16be = match_literal(<<$a:16,$b:16,$c:16>>),