column_specs: pubify and rename (de)constructors

Users might want to construct it manually, or convert `ColumnSpecs`
back to raw slice.

Author: muzarski

scylla/src/client/pager.rs

@@ -646,7 +646,7 @@ impl QueryPager {
     pub fn type_check<'frame, 'metadata, RowT: DeserializeRow<'frame, 'metadata>>(
         &self,
     ) -> Result<(), TypeCheckError> {
-        RowT::type_check(self.column_specs().inner())
+        RowT::type_check(self.column_specs().as_slice())
     }
 
     /// Casts the iterator to a given row type, enabling [Stream]'ed operations

scylla/src/response/query_result.rs

@@ -18,11 +18,13 @@ pub struct ColumnSpecs<'slice, 'spec> {
 }
 
 impl<'slice, 'spec> ColumnSpecs<'slice, 'spec> {
-    pub(crate) fn new(specs: &'slice [ColumnSpec<'spec>]) -> Self {
+    /// Creates new [`ColumnSpecs`] wrapper from a slice.
+    pub fn new(specs: &'slice [ColumnSpec<'spec>]) -> Self {
         Self { specs }
     }
 
-    pub(crate) fn inner(&self) -> &'slice [ColumnSpec<'spec>] {
+    /// Returns a slice of col specs encompassed by this struct.
+    pub fn as_slice(&self) -> &'slice [ColumnSpec<'spec>] {
         self.specs
     }