Replace `sub` with `delete_prefix`/`delete_suffix`

Ruby 2.5 introduced `delete_prefix` and `delete_suffix`.

Those methods are helpful when serializing formula and array formula
values, that are supposed to start and end with given prefixes

Also moves formula prefix to constants so they can be used by both
`Cell` and `CellSerializer` classes

Formula:
```
Ruby version: 3.2.2
Comparison:
       delete_prefix:  8759353.5 i/s
                 sub:  2607022.4 i/s - 3.36x  (± 0.00) slower

Comparison:
       delete_prefix:         40 allocated
                 sub:        160 allocated - 4.00x more

```

Array Formula:
```
Ruby version: 3.2.2
Comparison:
     delete_prefixes:  4798837.8 i/s
             sub_sub:   937072.1 i/s - 5.12x  (± 0.00) slower

Comparison:
     delete_prefixes:        120 allocated
             sub_sub:        488 allocated - 4.07x more
```

3 files changed, 14 insertions, 14 deletions

diff --git a/lib/axlsx/util/constants.rb b/lib/axlsx/util/constants.rb
index 4697bb3a..42a7683c 100644
--- a/lib/axlsx/util/constants.rb
+++ b/lib/axlsx/util/constants.rb
@@ -413,4 +413,14 @@ module Axlsx
 
   # Numeric recognition
   NUMERIC_REGEX = /\A[+-]?\d+?\Z/.freeze
+
+  # Leading characters that indicate a formula.
+  # See: https://owasp.org/www-community/attacks/CSV_Injection
+  FORMULA_PREFIX = '='
+
+  # Leading characters that indicate an array formula.
+  ARRAY_FORMULA_PREFIX = '{='
+
+  # Trailing character that indicates an array formula.
+  ARRAY_FORMULA_SUFFIX = '}'
 end
diff --git a/lib/axlsx/workbook/worksheet/cell.rb b/lib/axlsx/workbook/worksheet/cell.rb
index 929a7c28..f47781f0 100644
--- a/lib/axlsx/workbook/worksheet/cell.rb
+++ b/lib/axlsx/workbook/worksheet/cell.rb
@@ -72,16 +72,6 @@ module Axlsx
     CELL_TYPES = [:date, :time, :float, :integer, :richtext,
                   :string, :boolean, :iso_8601, :text].freeze
 
-    # Leading characters that indicate a formula.
-    # See: https://owasp.org/www-community/attacks/CSV_Injection
-    FORMULA_PREFIXES = ['='].freeze
-
-    # Leading characters that indicate an array formula.
-    ARRAY_FORMULA_PREFIXES = ['{='].freeze
-
-    # Trailing character that indicates an array formula.
-    ARRAY_FORMULA_SUFFIX = '}'
-
     # The index of the cellXfs item to be applied to this cell.
     # @return [Integer]
     # @see Axlsx::Styles
@@ -395,14 +385,14 @@ module Axlsx
     def is_formula?
       return false if escape_formulas
 
-      type == :string && @value.to_s.start_with?(*FORMULA_PREFIXES)
+      type == :string && @value.to_s.start_with?(FORMULA_PREFIX)
     end
 
     def is_array_formula?
       return false if escape_formulas
 
       type == :string &&
-        @value.to_s.start_with?(*ARRAY_FORMULA_PREFIXES) &&
+        @value.to_s.start_with?(ARRAY_FORMULA_PREFIX) &&
         @value.to_s.end_with?(ARRAY_FORMULA_SUFFIX)
     end
 
diff --git a/lib/axlsx/workbook/worksheet/cell_serializer.rb b/lib/axlsx/workbook/worksheet/cell_serializer.rb
index e1bdf728..9d39add3 100644
--- a/lib/axlsx/workbook/worksheet/cell_serializer.rb
+++ b/lib/axlsx/workbook/worksheet/cell_serializer.rb
@@ -88,7 +88,7 @@ module Axlsx
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
       def formula_serialization(cell, str = +'')
-        str << 't="str"><f>' << cell.clean_value.to_s.sub('=', '') << '</f>'
+        str << 't="str"><f>' << cell.clean_value.to_s.delete_prefix(FORMULA_PREFIX) << '</f>'
         str << '<v>' << cell.formula_value.to_s << '</v>' unless cell.formula_value.nil?
       end
 
@@ -97,7 +97,7 @@ module Axlsx
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
       def array_formula_serialization(cell, str = +'')
-        str << 't="str">' << '<f t="array" ref="' << cell.r << '">' << cell.clean_value.to_s.sub('{=', '').sub(/}$/, '') << '</f>'
+        str << 't="str">' << '<f t="array" ref="' << cell.r << '">' << cell.clean_value.to_s.delete_prefix(ARRAY_FORMULA_PREFIX).delete_suffix(ARRAY_FORMULA_SUFFIX) << '</f>'
         str << '<v>' << cell.formula_value.to_s << '</v>' unless cell.formula_value.nil?
       end